How TCP/IP Works

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

How TCP/IP Works

In this section

• TCP/IP Protocol Architecture

• IPv4 Addressing

• Name Resolution

• IPv4 Routing

• Physical Address Resolution

• Related Information

TCP/IP for IP version 4 (IPv4) is a networking protocol suite that Microsoft Windows uses to communicate over the internet with other computers. It interacts with Windows naming services like DNS and security technologies, such as IPsec primarily, as these help facilitate the successful and secure transfer of IP packets between machines.

Ideally, TCP/IP is used whenever Windows-based computers communicate over networks.

This subject describes the components of the TCP/IP Protocol Suite, the protocol architecture, which functions TCP/IP performs, how addresses are structured and assigned, and how packets are structured and routed.

Microsoft Windows Server 2003 provides extensive support for the Transmission Control Protocol/Internet Protocol (TCP/IP) suite, as both a protocol and a set of services for connectivity and management of IP internetworks. Knowledge of the basic concepts of TCP/IP is an absolute requirement for the proper understanding of the configuration, deployment, and troubleshooting of IP-based Windows Server 2003 and Microsoft Windows 2000 intranets.

TCP/IP Protocol Architecture

TCP/IP protocols map to a four-layer conceptual model known as the DARPA model, named after the U.S. government agency that initially developed TCP/IP. The four layers of the DARPA model are: Application, Transport, Internet, and Network Interface. Each layer in the DARPA model corresponds to one or more layers of the seven-layer Open Systems Interconnection (OSI) model.

The following figure shows the TCP/IP protocol architecture.

TCP/IP Protocol Architecture

Note

• The architectural diagram above corresponds to the Internet Protocol TCP/IP and does not reflect IP version 6. To see a TCP/IP architectural diagrm that includes IPv6, see How IPv6 Works in this technical reference.

Network Interface Layer

The Network Interface layer (also called the Network Access layer) handles placing TCP/IP packets on the network medium and receiving TCP/IP packets off the network medium. TCP/IP was designed to be independent of the network access method, frame format, and medium. In this way, TCP/IP can be used to connect differing network types. These include local area network (LAN) media such as Ethernet and Token Ring and WAN technologies such as X.25 and Frame Relay. Independence from any specific network media allows TCP/IP to be adapted to new media such as asynchronous transfer mode (ATM).

The Network Interface layer encompasses the Data Link and Physical layers of the OSI model. Note that the Internet layer does not take advantage of sequencing and acknowledgment services that might be present in the Network Interface layer. An unreliable Network Interface layer is assumed, and reliable communication through session establishment and the sequencing and acknowledgment of packets is the function of the Transport layer.

Internet Layer

The Internet layer handles addressing, packaging, and routing functions. The core protocols of the Internet layer are IP, ARP, ICMP, and IGMP.

• The Internet Protocol (IP) is a routable protocol that handles IP addressing, routing, and the fragmentation and reassembly of packets.

• The Address Resolution Protocol (ARP) handles resolution of an Internet layer address to a Network Interface layer address, such as a hardware address.

• The Internet Control Message Protocol (ICMP) handles providing diagnostic functions and reporting errors due to the unsuccessful delivery of IP packets.

• The Internet Group Management Protocol (IGMP) handles management of IP multicast group membership.

The Internet layer is analogous to the Network layer of the OSI model.

Transport Layer

The Transport layer (also known as the Host-to-Host Transport layer) handles providing the Application layer with session and datagram communication services. The core protocols of the Transport layer are Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

• TCP provides a one-to-one, connection-oriented, reliable communications service. TCP handles the establishment of a TCP connection, the sequencing and acknowledgment of packets sent, and the recovery of packets lost during transmission.

• UDP provides a one-to-one or one-to-many, connectionless, unreliable communications service. UDP is used when the amount of data to be transferred is small (such as data that fits into a single packet), when you do not want the overhead of establishing a TCP connection, or when the applications or upper layer protocols provide reliable delivery.

The TCP/IP Transport layer encompasses the responsibilities of the OSI Transport layer.

Application Layer

The Application layer lets applications access the services of the other layers and defines the protocols that applications use to exchange data. There are many Application layer protocols and new protocols are always being developed.

The most widely known Application layer protocols are those used for the exchange of user information:

• The Hypertext Transfer Protocol (HTTP) is used to transfer files that make up the Web pages of the World Wide Web.

• The File Transfer Protocol (FTP) is used for interactive file transfer.

• The Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and attachments.

• Telnet, a terminal emulation protocol, is used for logging on remotely to network hosts.

Additionally, the following Application layer protocols help facilitate the use and management of TCP/IP networks:

• The Domain Name System (DNS) is used to resolve a host name to an IP address.

• The Routing Information Protocol (RIP) is a routing protocol that routers use to exchange routing information on an IP internetwork.

• The Simple Network Management Protocol (SNMP) is used between a network management console and network devices (routers, bridges, intelligent hubs) to collect and exchange network management information.

Examples of Application layer interfaces for TCP/IP applications are Windows Sockets and NetBIOS. Windows Sockets provides a standard application programming interface (API) under Windows Server 2003. NetBIOS is an industry-standard interface for accessing protocol services such as sessions, datagrams, and name resolution. More information on Windows Sockets and NetBIOS is provided later in this chapter.

The TCP/IP Application layer encompasses the responsibilities of the OSI Session, Presentation, and Application layers.

TCP/IP Core Protocols

The TCP/IP protocol component that is installed in your network operating system is a series of interconnected protocols called the core protocols of TCP/IP. All other applications and other protocols in the TCP/IP protocol suite rely on the basic services provided by the following protocols: IP, ARP, ICMP, IGMP, TCP, and UDP.

IP

IP is a connectionless, unreliable datagram protocol primarily responsible for addressing and routing packets between hosts. Connectionless means that a session is not established before exchanging data. Unreliable means that delivery is not guaranteed. IP always makes a “best effort” attempt to deliver a packet. An IP packet might be lost, delivered out of sequence, duplicated, or delayed. IP does not attempt to recover from these types of errors. The acknowledgment of packets delivered and the recovery of lost packets is the responsibility of a higher-layer protocol, such as TCP. IP is defined in RFC 791.

An IP packet consists of an IP header and an IP payload. The following table describes the key fields in the IP header.

Key Fields in the IP Header

IP Header Field	Function
Source Address	The IP address of the original source of the IP datagram.
Destination Address	The IP address of the final destination of the IP datagram.
Identification	Used to identify a specific IP datagram and to identify all fragments of a specific IP datagram if fragmentation occurs.
Protocol	Informs IP at the destination host whether to pass the packet up to TCP, UDP, ICMP, or other protocols.
Checksum	A simple mathematical computation used to verify the bit-level integrity of the IP header.
Time to Live (TTL)	Designates the number of network segments on which the datagram is allowed to travel before being discarded by a router. The TTL is set by the sending host and is used to prevent packets from endlessly circulating on an IP internetwork. When forwarding an IP packet, routers are required to decrease the TTL by at least one.

Fragmentation and reassembly

If a router receives an IP packet that is too large for the network to which the packet is being forwarded, IP fragments the original packet into smaller packets that fit on the downstream network. When the packets arrive at their final destination, IP on the destination host reassembles the fragments into the original payload. This process is referred to as fragmentation and reassembly. Fragmentation can occur in environments that have a mix of networking media, such as Ethernet and Token Ring.

The fragmentation and reassembly works as follows:

• When an IP packet is sent by the source, it places a unique value in the Identification field.

• The IP packet is received at the router. The IP router notes that the maximum transmission unit (MTU) of the network onto which the packet is to be forwarded is smaller than the size of the IP packet.

• IP divides the original IP payload into fragments that fit on the next network. Each fragment is sent with its own IP header that contains:

  • The original Identification field identifying all fragments that belong together.

  • The More Fragments Flag indicating that other fragments follow. The More Fragments Flag is not set on the last fragment, because no other fragments follow it.

  • The Fragment Offset field indicating the position of the fragment relative to the original IP payload.

When the fragments are received by IP at the remote host, they are identified by the Identification field as belonging together. The Fragment Offset field is then used to reassemble the fragments into the original IP payload.

ARP

When IP packets are sent on shared access, broadcast-based networking media — such as Ethernet or Token Ring — the media access control (MAC) address corresponding to a forwarding IP address must be resolved. ARP uses MAC-level broadcasts to resolve a known forwarding or next-hop IP address to its MAC address. ARP is defined in RFC 826.

ICMP

Internet Control Message Protocol (ICMP) provides troubleshooting facilities and error reporting for packets that are undeliverable. For example, if IP is unable to deliver a packet to the destination host, ICMP sends a Destination Unreachable message to the source host. The following table shows the most common ICMP messages.

Common ICMP Messages

ICMP Message	Function
Echo Request	Troubleshooting message used to check IP connectivity to a desired host. The ping utility sends ICMP Echo Request messages.
Echo Reply	Response to an ICMP Echo Request.
Redirect	Sent by a router to inform a sending host of a better route to a destination IP address.
Source Quench	Sent by a router to inform a sending host that its IP datagrams are being dropped due to congestion at the router. The sending host then lowers its transmission rate. Source Quench is an elective ICMP message and is not commonly implemented.
Destination Unreachable	Sent by a router or the destination host to inform the sending host that the datagram cannot be delivered.

The following table describes the most common ICMP Destination Unreachable ICMP messages.

Common ICMP Destination Unreachable Messages

Destination Unreachable Message	Description
Host Unreachable	Sent by an IP router when a route to the destination IP address cannot be found.
Protocol Unreachable	Sent by the destination IP node when the Protocol field in the IP header cannot be matched with an IP client protocol currently loaded.
Port Unreachable	Sent by the destination IP node when the Destination Port in the UDP header cannot be matched with a process using that port.
Fragmentation Needed and DF Set	Sent by an IP router when fragmentation must occur but is not allowed due to the source node setting the Don’t Fragment (DF) flag in the IP header.
Source Route Failed	Sent by an IP router when delivery of the IP packet using source route information (stored as source route option headers) fails.

ICMP does not make IP a reliable protocol. ICMP attempts to report errors and provide feedback on specific conditions. ICMP messages are carried as unacknowledged IP datagrams and are themselves unreliable. ICMP is defined in RFC 792.

IGMP

Internet Group Management Protocol (IGMP) is a protocol that manages host membership in IP multicast groups on a network segment. An IP multicast group, also known as a host group, is a set of hosts that listen for IP traffic destined for a specific IP multicast address. IP multicast traffic is sent to a single MAC address but processed by multiple IP hosts. A specific host listens on a specific IP multicast address and receives all packets to that IP address.

The following are some of the additional aspects of IP multicasting:

• Host group membership is dynamic, hosts can join and leave the group at any time.

• A host group can be of any size.

• Members of a host group can span IP routers across multiple networks. This situation requires IP multicast support on the IP routers and the ability for hosts to register their group membership with local routers. Host registration is accomplished using IGMP.

• A host can send traffic to an IP multicast address without belonging to the corresponding host group.

For a host to receive IP multicasts, an application must inform IP that it will receive multicasts at a specified IP multicast address. If the network technology supports hardware-based multicasting, the network interface is told to pass up packets for a specific IP multicast address. In the case of Ethernet, the network adapter is programmed to respond to a multicast MAC address corresponding to the specified IP multicast address.

A host supports IP multicast at one of the following levels:

• Level 0: No support to send or receive IP multicast traffic.

• Level 1: Support exists to send but not receive IP multicast traffic.

• Level 2: Support exists to both send and receive IP multicast traffic. Windows Server 2003, Windows 2000, Microsoft Windows NT version 3.5 and later, and TCP/IP support level 2 IP multicasting.

The protocol to register host group information is IGMP, which is required on all hosts that support level 2 IP multicasting. IGMP packets are sent using an IP header.

IGMP messages take three forms.

• Host Membership Report. When a host joins a host group, it sends an IGMP Host Membership Report message to the all-hosts IP multicast address (224.0.0.1) or to the specified IP multicast address declaring its membership in a specific host group by referencing the IP multicast address. A host can also specify the specific sources from which multicast traffic is needed.

• Host Membership Query. When a router polls a network to ensure that there are members of a specific host group, it sends an IGMP Host Membership Query message to the all-hosts IP multicast address. If no responses to the poll are received after several polls, the router assumes no membership in that group for that network and stops advertising that multicast group information to other routers.

• Group Leave. When a host is no longer interested in receiving multicast traffic sent to a specific IP multicast address and it sent the last IGMP Host Membership Report message in response to an IGMP Host Membership Query, it sends an IGMP Group Leave message to the specific IP multicast address. Local routers verify that the host sending the IGMP Group Leave message is the last group member for that multicast address on that subnet. If no responses to the poll are received after several polls, the router assumes no membership in that group for that subnet and stops advertising that multicast group information to other routers.

For IP multicasting to span routers across an internetwork, multicast routing protocols are used by routers to communicate host group information so that each router supporting multicast forwarding is aware of which networks contain members of which host groups. IGMP is defined in RFCs 1112 and 2236.

TCP

TCP is a reliable, connection-oriented delivery service. The data is transmitted in segments. Connection-oriented means that a connection must be established before hosts can exchange data. Reliability is achieved by assigning a sequence number to each segment transmitted. An acknowledgment is used to verify that the data is received. For each segment sent, the receiving host must return an acknowledgment (ACK) within a specified period for bytes received. If an ACK is not received, the data is retransmitted. TCP is defined in RFC 793.

TCP uses byte-stream communications, wherein data within the TCP segment is treated as a sequence of bytes with no record or field boundaries. The following table describes the key fields in the TCP header.

Key Fields in the TCP Header

Field	Function
Source Port	TCP port of sending host.
Destination Port	TCP port of destination host.
Sequence Number	Sequence number of the first byte of data in the TCP segment.
Acknowledgment Number	Sequence number of the byte the sender expects to receive next from the other side of the connection.
Window	Current size of a TCP buffer on the host sending this TCP segment to store incoming segments.
TCP Checksum	Verifies the bit-level integrity of the TCP header and the TCP data.

TCP ports

A TCP port provides a specific location for delivery of TCP segments. Port numbers below 1024 are well-known ports and are assigned by the Internet Assigned Numbers Authority (IANA). The following table lists a few well-known TCP ports.

Well-Known TCP Ports

TCP Port Number	Description
20	FTP (Data Channel)
21	FTP (Control Channel)
23	Telnet
80	HTTP used for the World Wide Web
139	NetBIOS session service

TCP three-way handshake

A TCP connection is initialized through a three-way handshake. The purpose of the three-way handshake is to synchronize the sequence number and acknowledgment numbers of both sides of the connection and exchange TCP window sizes or the use of large window sizes or TCP timestamps. The following steps outline the process:

1. The initiator of the TCP connection, typically a client, sends a TCP segment to the server with an initial Sequence Number for the connection and a window size indicating the size of a buffer on the client to store incoming segments from the server.

2. The responder of the TCP connection, typically a server, sends back a TCP segment containing its chosen initial Sequence Number, an acknowledgment of the client’s Sequence Number, and a window size indicating the size of a buffer on the server to store incoming segments from the client.

3. The initiator sends a TCP segment to the server containing an acknowledgment of the server’s Sequence Number.

TCP uses a similar handshake process to end a connection. This guarantees that both hosts have finished transmitting and that all data was received.

UDP

UDP provides a connectionless datagram service that offers unreliable, best-effort delivery of data transmitted in messages. This means that neither the arrival of datagrams nor the correct sequencing of delivered packets is guaranteed. UDP does not recover from lost data through retransmission. UDP is defined in RFC 768.

UDP is used by applications that do not require an acknowledgment of receipt of data and that typically transmit small amounts of data at one time. NetBIOS name service, NetBIOS datagram service, and SNMP are examples of services and applications that use UDP. The following table describes the key fields in the UDP header.

Key Fields in the UDP Header

Field	Function
Source Port	UDP port of sending host.
Destination Port	UDP port of destination host.
UDP Checksum	Verifies the bit-level integrity of the UDP header and the UDP data.

UDP ports

To use UDP, an application must supply the IP address and UDP port number of the destination application. A port provides a location for sending messages. A port functions as a multiplexed message queue, meaning that it can receive multiple messages at a time. Each port is identified by a unique number. It is important to note that UDP ports are distinct and separate from TCP ports even though some of them use the same number. The following table lists a few well-known UDP ports.

Well-Known UDP Ports

UDP Port Number	Description
53	Domain Name System (DNS) name queries
69	Trivial File Transfer Protocol (TFTP)
137	NetBIOS name service
138	NetBIOS datagram service
161	SNMP

TCP/IP Application Interfaces

For applications to access the services offered by the core TCP/IP protocols in a standard way, network operating systems like Windows Server 2003 make industry-standard application programming interfaces (APIs) available. APIs are sets of functions and commands that are programmatically called by application code to perform network functions. For example, a Web browser application connecting to a Web site needs access to TCP’s connection establishment service.

The following figure shows two common TCP/IP APIs, Windows Sockets and NetBIOS, and their relation to the core protocols.

APIs for TCP/IP

Windows Sockets Interface

The Windows Sockets API is a standard API under Windows Server 2003 for applications that use TCP and UDP. Applications written to the Windows Sockets API run on many versions of TCP/IP. TCP/IP utilities and the SNMP service are examples of applications written to the Windows Sockets interface.

Windows Sockets provides services that allow applications to bind to a particular port and IP address on a host, initiate and accept a connection, send and receive data, and close a connection. There are two types of sockets:

• A stream socket provides a two-way, reliable, sequenced, and unduplicated flow of data using TCP.

• A datagram socket provides a one-way or two-way flow of data using UDP.

A socket is defined by a protocol and an address on the host. The format of the address is specific to each protocol. In TCP/IP, the address is the combination of the IP address and port. Two sockets, one for each end of the connection, form a bi-directional communications path.

To communicate, an application specifies the protocol, the IP address of the destination host, and the port of the destination application. After the application is connected, information can be sent and received.

NetBIOS Interface

NetBIOS allows applications to communicate over a network. NetBIOS defines two entities, a session-level interfaceand a session management and data transport protocol.

The NetBIOS interface is a standard API for user applications to submit network input/output (I/O) and control directives to underlying network protocol software. An application program that uses the NetBIOS interface API for network communication can be run on any protocol software that supports the NetBIOS interface.

NetBIOS also defines a protocolthat functions at thesession/transport level. This is implemented by the underlying protocol software (such as the NetBIOS Frames Protocol NBFP — a component of NetBEUI or NetBIOS over TCP/IP (NetBT)), which performs the network I/O required to accommodate the NetBIOS interface command set. NetBIOS over TCP/IP is defined in RFCs 1001 and 1002. NetBT is enabled by default, however Windows Server 2003 allows you to disable NetBT for an environment that contains no NetBIOS-based network clients or applications.

NetBIOS provides commands and support for NetBIOS Name Management, NetBIOS Datagrams, and NetBIOS Sessions.

NetBIOS name management

NetBIOS name management services provide the following functions:

• Name registration and release. When a TCP/IP host initializes, it registers its NetBIOS names by broadcasting or directing a NetBIOS name registration request to a NetBIOS Name Server such as a WINS server. If another host has registered the same NetBIOS name, either the host or a NetBIOS Name Server responds with a negative name registration response. The initiating host receives an initialization error as a result. When the workstation service on a host is stopped, the host discontinues broadcasting a negative name registration response when someone else tries to use the name and sends a name release to a NetBIOS Name Server. The NetBIOS name is said to be released and available for use by another host.

• Name Resolution. When a NetBIOS application wants to communicate with another NetBIOS application, the IP address of the NetBIOS application must be resolved. NetBT performs this function by either broadcasting a NetBIOS name query on the local network or sending a NetBIOS name query to a NetBIOS Name Server.

The NetBIOS name service uses UDP port 137.

NetBIOS datagrams

The NetBIOS datagram service provides delivery of datagrams that are connectionless, unsequenced, and unreliable. Datagrams can be directed to a specific NetBIOS name or broadcast to a group of names. Delivery is unreliable in that only the users who are logged on to the network receive the message. The datagram service can initiate and receive both broadcast and directed messages. The NetBIOS datagram service uses UDP port 138.

NetBIOS sessions

The NetBIOS session service provides delivery of NetBIOS messages that are connection-oriented, sequenced, and reliable. NetBIOS sessions use TCP connections and provide session establishment, keepalive, and termination. The NetBIOS session service allows concurrent data transfers in both directions using TCP port 139.

IPv4 Addressing

For IP version 4, each TCP/IP host is identified by a logical IP address. The IP address is a Network layer address and has no dependence on the Data-Link layer address (such as a MAC address of a network adapter). A unique IP address is required for each host and network component that communicates using TCP/IP and can be assigned manually or by using Dynamic Host Configuration Protocol (DHCP).

The IP address identifies a system’s location on the network in the same way a street address identifies a house on a city block. Just as a street address must identify a unique residence, an IP address must be globally unique to the internetwork and have a uniform format.

Each IP address includes a network ID and a host ID.

• The network ID (also known as a network address) identifies the systems that are located on the same physical network bounded by IP routers. All systems on the same physical network must have the same network ID. The network ID must be unique to the internetwork.

• The host ID (also known as a host address) identifies a workstation, server, router, or other TCP/IP host within a network. The host address must be unique to the network ID.

IPv4 Address Syntax

An IP address consists of 32 bits. Instead of expressing IPv4 addresses 32 bits at a time using binary notation (Base2), it is standard practice to segment the 32 bits of an IPv4 address into four 8-bit fields called octets. Each octet is converted to a decimal number (base 10) from 0–255 and separated by a period (a dot). This format is called dotted decimal notation. The following table provides an example of an IP address in binary and dotted decimal formats.

An IP Address in Binary and Dotted Decimal Formats

Binary Format	Dotted Decimal Notation
11000000 10101000 00000011 00011000	192.168.3.24

For example, the IPv4 address of 11000000101010000000001100011000 is:

• Segmented into 8-bit blocks: 11000000 10101000 00000011 00011000.

• Each block is converted to decimal: 192 168 3 24

• The adjacent octets are separated by a period: 192.168.3.24.

The notation w.x.y.z is used when referring to a generalized IP address, and is shown the following figure.

IP Address

Types of IPv4 Addresses

The Internet standards define the following types of IPv4 addresses:

• Unicast. Assigned to a single network interface located on a specific subnet on the network and used for one-to-one communications.

• Multicast. Assigned to one or more network interfaces located on various subnets on the network and used for one-to-many communications.

• Broadcast. Assigned to all network interfaces located on a subnet on the network and used for one-to-everyone-on-a-subnet communications.

The following sections describe these types of addresses in detail.

IPv4 Unicast Addresses

The IPv4 unicast address identifies an interface’s location on the network in the same way a street address identifies a house on a city block. Just as a street address must identify a unique residence, an IPv4 unicast address must be globally unique to the network and have a uniform format.

Each IPv4 unicast address includes a network ID and a host ID.

• The network ID (also known as a network address) is the fixed portion of an IPv4 unicast address that identifies the set of interfaces that are located on the same physical or logical network segment as bounded by IPv4 routers. A network segment on TCP/IP networks is also known as a subnet. All systems on the same physical or logical subnet must use the same network ID and the network ID must be unique to the entire TCP/IP network.

• The host ID (also known as a host address) is the variable portion of an IPv4 unicast address that is used to identify a network node’s interface on a subnet. The host ID must be unique to the network ID.

If the network ID is unique to the TCP/IP network and the host ID is unique to the network ID, then the entire IPv4 unicast address consisting of the network ID and host ID is unique to the entire TCP/IP network.

IPv4 Multicast Addresses

IPv4 multicast addresses are used for single-packet one-to-many delivery. On an IPv4 multicast-enabled intranet, an IPv4 packet addressed to an IPv4 multicast address is forwarded by routers to the subnets on which there are hosts listening to the traffic sent to the IPv4 multicast address. IPv4 multicast provides an efficient one-to-many delivery service for many types of communication.

IPv4 multicast addresses are defined by the class D Internet address class: 224.0.0.0/4. IPv4 multicast addresses range from 224.0.0.0 through 239.255.255.255. IPv4 multicast addresses for the 224.0.0.0/24 address prefix (224.0.0.0 through 224.0.0.255) are reserved for local subnet multicast traffic.

IPv4 Broadcast Addresses

IPv4 uses a set of broadcast addresses to provide a one-to-everyone on the subnet delivery service. Packets sent to IPv4 broadcast addresses are processed by all the interfaces on the subnet. The following are the different types of IPv4 broadcast addresses:

• Network broadcast. Formed by setting all the host bits to 1 for a classful address prefix. An example of a network broadcast address for the classful network ID 131.107.0.0/16 is 131.107.255.255. Network broadcasts are used to send packets to all interfaces of a classful network. IPv4 routers do not forward network broadcast packets.

• Subnet broadcast. Formed by setting all the host bits to 1 for a classless address prefix. An example of a network broadcast address for the classless network ID 131.107.26.0/24 is 131.107.26.255. Subnet broadcasts are used to send packets to all hosts of a classless network. IPv4 routers do not forward subnet broadcast packets. For a classful address prefix, there is no subnet broadcast address, only a network broadcast address. For a classless address prefix, there is no network broadcast address, only a subnet broadcast address.

• All-subnets-directed broadcast. Formed by setting all the original classful network ID host bits to 1 for a classless address prefix. A packet addressed to the all-subnets-directed broadcast was defined to reach all hosts on all of the subnets of a subnetted class-based network ID. An example of an all-subnets-directed broadcast address for the subnetted network ID 131.107.26.0/24 is 131.107.255.255. The all-subnets-directed broadcast is the network broadcast address of the original classful network ID. IPv4 routers can forward all-subnets directed broadcast packets, however the use of the all-subnets-directed broadcast address is deprecated in RFC 1812.

• Limited broadcast. Formed by setting all 32 bits of the IPv4 address to 1 (255.255.255.255). The limited broadcast address is used for one-to-everyone delivery on the local subnet when the local network ID is unknown. IPv4 nodes typically only use the limited broadcast address during an automated configuration process such as Boot Protocol (BOOTP) or DHCP. For example, with DHCP, a DHCP client must use the limited broadcast address for all traffic sent until the DHCP server acknowledges the use of the offered IPv4 address configuration. IPv4 routers do not forward limited broadcast packets.

Internet Address Classes

The Internet community originally defined address classes to accommodate different types of addresses and networks of varying sizes. The class of address defined which bits were used for the network ID and which bits were used for the host ID. It also defined the possible number of networks and the number of hosts per network. Of five address classes, class A, B, and C addresses were defined for IPv4 unicast addresses. Class D addresses were defined for IPv4 multicast addresses and class E addresses were defined for experimental uses.

Class A

Class A network IDs were assigned to networks with a very large number of hosts. The high-order bit in a class A address is always set to zero, which makes the address prefix for all class A networks and addresses 0.0.0.0/1 (or 0.0.0.0, 128.0.0.0). The next seven bits (completing the first octet) are used to enumerate class A network IDs. Therefore, address prefixes for class A network IDs have an 8-bit prefix length (/8 or 255.0.0.0). The remaining 24 bits (the last three octets) are used for the host ID. The address prefix 0.0.0.0/0 (or 0.0.0.0, 0.0.0.0) is a reserved network ID and 127.0.0.0/8 (or 127.0.0.0, 255.0.0.0) is reserved for loopback addresses. Out of a total of 128 possible class A networks, there are 126 networks and 16,777,214 hosts per network.

Note

• All-Zeros and All-Ones Host IDs are Reserved

• When enumerating host IDs for a given network ID, the two host IDs in which all the bits in the host ID are set to 0 (the all-zeros host ID) and all the bits in the host ID is set to 1 (the all-ones host ID) are reserved and cannot be assigned to network node interfaces. Hence, in the calculation above in which there are 24 bits for class A host IDs, the total number of possible host IDs is 16,777,216 (224). When you subtract the two reserved host IDs, the total number of usable host IDs is 16,777,214.

The following figure illustrates the structure of class A addresses.

Structure of class A addresses

Class B

Class B network IDs were assigned to medium to large-sized networks. The two high-order bits in a class B address are always set to 10, which makes the address prefix for all class B networks and addresses 128.0.0.0/2 (or 128.0.0.0, 192.0.0.0). The next 14 bits (completing the first two octets) are used to enumerate class B network IDs. Therefore, address prefixes for class B network IDs have a 16-bit prefix length (/16 or 255.255.0.0). The remaining 16 bits (last two octets) are used for the host ID. With 14 bits to express class B network IDs and 16 bits to express host IDs, this allows for 16,384 networks and 65,534 hosts per network.

The following figure illustrates the structure of class B addresses.

Structure of class B addresses

Class C

Class C addresses were assigned to small networks. The three high-order bits in a class C address are always set to 110, which makes the address prefix for all class C networks and addresses 192.0.0.0/3 (or 192.0.0.0, 224.0.0.0). The next 21 bits (completing the first three octets) are used to enumerate class C network IDs. Therefore, address prefixes for class C network IDs have a 24-bit prefix length (/24 or 255.255.255.0). The remaining 8 bits (the last octet) are used for the host ID. With 21 bits to express class C network IDs and 8 bits to express host IDs, this allows for 2,097,152 networks and 254 hosts per network.

The following figure illustrates the structure of class C addresses.

Structure of class C addresses

Class D

Class D addresses are reserved for IPv4 multicast addresses. The four high-order bits in a class D address are always set to 1110, which makes the address prefix for all class D addresses 224.0.0.0/4 (or 224.0.0.0, 240.0.0.0).

Class E

Class E addresses are reserved for experimental use. The high-order bits in a class E address are set to 1111, which makes the address prefix for all class E addresses 240.0.0.0/4 (or 240.0.0.0, 240.0.0.0)

The following table is a summary of the Internet address classes A, B, and C that can be used for IPv4 unicast addresses.

Internet Address Class Summary

Class	Value for w	Network ID Portion	Host ID Portion	Network IDs	Host IDs per Network
A	1-126	w	x.y.z	126	16,777,214
B	128-191	w.x	y.z	16,384	65,534
C	192-223	w.x.y	z	2,097,152	254

Modern Internet Addresses

The Internet address classes are an obsolete unicast address allocation method that proved to be an inefficient way to assign network IDs and addresses to organizations connected to the Internet. For example, a large organization with a class A network ID can have up to 16,777,214 hosts. However, if the organization only uses 70,000 host IDs, then 16,707,214 potential IPv4 unicast addresses for the Internet are wasted.

On the modern-day Internet, IPv4 address prefixes are handed out to organization’s based on the organization’s actual need for Internet-accessible IPv4 unicast addresses using a method known as Classless Inter-Domain Routing (CIDR). For example, an organization determines that it needs 2,000 Internet-accessible IPv4 unicast addresses. The Internet Corporation for Assigned Names and Numbers (ICANN) or an Internet service provider (ISP) allocates an IPv4 address prefix in which 21 bits are fixed, leaving 11 bits for host IDs. From the 11 bits for host IDs, the organization can create 2,032 possible IPv4 unicast addresses.

CIDR-based address allocations typically start at 8 bits. The following table lists the required number of host IDs and the corresponding prefix length for CIDR-based address allocations.

Host IDs Needed and CIDR-based Prefix Lengths

Number of Host IDs	Prefix Length	Dotted Decimal
2–254	/24	255.255.255.0
255–510	/23	255.255.254.0
511–1,022	/22	255.255.252.0
1,021–2,046	/21	255.255.248.0
2,047–4,094	/20	255.255.240.0
4,095–8,190	/19	255.255.224.0
8,191–16,382	/18	255.255.192.0
16,383–32,766	/17	255.255.128.0
32,767–65,534	/16	255.255.0.0

Public and Private Addresses

If you want direct (routed) connectivity to the Internet, then you must use public addresses. If you want indirect (proxied or translated) connectivity to the Internet, you can use either public or private addresses. If your intranet is not connected to the Internet in any way, you can use any unicast IPv4 addresses that you want. However, you should use private addresses to avoid network renumbering when your intranet is eventually connected to the Internet.

Public addresses

Public addresses are assigned by ICANN and consist of either historically allocated class-based network IDs or, more recently, CIDR-based address prefixes that are guaranteed to be globally unique on the Internet. For CIDR-based address prefixes, the value of w (the first octet) is in the ranges of 1 through 126 and 128 through 223, with the exception of the private address prefixes described in “Private Addresses.”

When the public addresses are assigned, routes are added to the routers of the Internet so that traffic sent to an address that matches the assigned public address prefix can reach the assigned organization. For example, when an organization is assigned an address prefix in the form of a network ID and prefix length, that address prefix also exists as a route in the routers of the Internet. IPv4 packets destined to an address within the assigned address prefix are routed to the proper destination.

Private addresses

Each IPv4 interface requires an IPv4 address that is globally unique to the IPv4 network. In the case of the Internet, each IPv4 interface on a subnet connected to the Internet requires an IPv4 address that is globally unique to the Internet. As the Internet grew, organizations connecting to the Internet required a public address for each interface on their intranets. This requirement placed a huge demand on the pool of available public addresses.

When analyzing the addressing needs of organizations, the designers of the Internet noted that for many organizations, most of the hosts on an organization’s intranet did not require direct connectivity to the Internet. Those hosts that did require a specific set of Internet services, such as Web access and e-mail, typically access the Internet services through Application layer gateways such as proxy servers and e-mail servers. The result is that most organizations only required a small amount of public addresses for those nodes (such as proxies, servers, routers, firewalls, and translators) that were directly connected to the Internet.

For the hosts within the organization that do not require direct access to the Internet, IPv4 addresses that do not duplicate already-assigned public addresses are required. To solve this addressing problem, the Internet designers reserved a portion of the IPv4 address space and named this space the private address space. An IPv4 address in the private address space is never assigned as a public address. IPv4 addresses within the private address space are known as private addresses. Because the public and private address spaces do not overlap, private addresses never duplicate public addresses.

The private address space specified in RFC 1918 is defined by the following address prefixes:

• 10.0.0.0/8 (10.0.0.0, 255.0.0.0)

  Allows the following range of valid IPv4 unicast addresses: 10.0.0.1 to 10.255.255.254. The 10.0.0.0/8 address prefix has 24 host bits that can be used for any addressing scheme within the private organization.

• 172.16.0.0/12 (172.16.0.0, 255.240.0.0)

  Allows the following range of valid IPv4 unicast addresses: 172.16.0.1 to 172.31.255.254. The 172.16.0.0/12 address prefix has 20 host bits that can be used for any addressing scheme within the private organization.

• 192.168.0.0/16 (192.168.0.0, 255.255.0.0)

  Allows the following range of valid IPv4 unicast addresses: 192.168.0.1 to 192.168.255.254. The 192.168.0.0/16 address prefix has 16 host bits that can be used for any addressing scheme within the private organization.

Because the IPv4 addresses in the private address space will never be assigned by ICANN to an organization connected to the Internet, there will never be routes for the private address prefixes in Internet routers. You cannot connect to a private address over the Internet. Therefore, a host that has a private address must send its Internet traffic requests to an Application layer gateway (such as a proxy server) that has a valid public address or through a network address translator (NAT) that translates the private address into a valid public address.

Illegal addresses

Private organization intranets that do not need an Internet connection can choose any address scheme they want, even using public address prefixes that have been assigned by ICANN. If that organization later decides to connect to the Internet, its current address scheme might include addresses already assigned by ICANN to other organizations. These addresses conflict with existing public addresses assigned by ICANN and are known as illegal addresses. Connectivity from illegal addresses to Internet locations is not possible because the routers of the Internet send traffic destined to ICANN-allocated address prefixes to the assigned organizations, not to the organizations using illegal addresses.

For example, a private organization chooses to use the 206.73.118.0/24 address prefix for its intranet. The public address prefix 206.73.118.0/24 has been assigned by ICANN to the Microsoft Corporation and routes exist on the Internet routers to send all packets for IPv4 addresses on 206.73.118.0/24 to Microsoft routers. As long as the private organization does not connect to the Internet, there is no problem because the two address prefixes are on separate IPv4 networks; therefore they are unique to each separate network. If the private organization later connects directly to the Internet and continues to use the 206.73.118.0/24 address prefix, any Internet response traffic to locations matching the 206.73.118.0/24 address prefix is sent to Microsoft routers, not to the routers of the private organization.

Automatic Private IP Addressing

An interface on a computer running Windows Server 2003 and Windows XP that is configured to obtain an IPv4 address configuration automatically that does not successfully contact a Dynamic Host Configuration Protocol (DHCP) server uses its alternate configuration, as specified on the Alternate Configuration tab.

If the Automatic Private IP Address option is selected on the Alternate Configuration tab and a DHCP server cannot be found, Windows TCP/IP uses Automatic Private IP Addressing (APIPA). Windows TCP/IP randomly selects an IPv4 address from the 169.254.0.0/16 address prefix and assigns the subnet mask of 255.255.0.0. This address prefix has been reserved by the ICANN and is not reachable on the Internet. APIPA allows single-subnet Small Office/Home Office (SOHO) networks to use TCP/IP without static configuration or the administration of a DHCP server. APIPA does not configure a default gateway. Therefore, only local subnet traffic is possible.

Special IPv4 Addresses

The following are special IPv4 addresses:

• 0.0.0.0

  Known as the unspecified IPv4 address, it is used to indicate the absence of an address. The unspecified address is used only as a source address when the IPv4 node is not configured with an IPv4 address configuration and is attempting to obtain an address through a configuration protocol such as Dynamic Host Configuration Protocol (DHCP).

• 127.0.0.1

  Known as the IPv4 loopback address, it is assigned to an internal loopback interface, enabling a node to send packets to itself.

Unicast IPv4 Addressing Guidelines

When assigning network IDs to the subnets of an organization, use the following guidelines:

• The network ID must be unique on the IPv4 network.

  If the network ID is for a subnet on which there are hosts that are directly accessible from the Internet, you must use a public IPv4 address prefix assigned by ICANN or an Internet service provider. If the network ID is for a subnet that is not directly accessible by the Internet, use either a legal public address prefix or a private address prefix that is unique on your private intranet.

• The network ID cannot begin with the numbers 0 or 127.

  Both of these values for the first octet are reserved and cannot be used for IPv4 unicast addresses.

When assigning host IDs to the interfaces of nodes on an IPv4 subnet, use the following guidelines:

• The host ID must be unique on the subnet.

• You cannot use the all-zeros or all-ones host IDs.

When defining the range of valid IPv4 unicast addresses for a given address prefix, use the following standard practice:

• For the first IPv4 unicast address in the range, set all the host bits in the address to 0, except for the low-order bit, which is set to 1.

• For the last IPv4 unicast address in the range, set all the host bits in the address to 1, except for the low-order bit, which is set to 0.

For example, to express the range of addresses for the address prefix 192.168.16.0/20:

• The first IPv4 unicast address in the range is 11000000 10101000 0001000000000001 (host bits are bold), or 192.168.16.1.

• The last IPv4 unicast address in the range is 11000000 10101000 0001111111111110 (host bits are bold), or 192.168.31.254.

Name Resolution

While IP is designed to work with the 32-bit IP addresses of the source and the destination hosts, computers users are much better at using and remembering names than IP addresses.

When a name is used as an alias for an IP address, a mechanism must exist for assigning that name to the appropriate IP node — to ensure its uniqueness and resolution to its IP address.

In this section, the mechanisms used for assigning and resolving host names (which are used by Windows Sockets applications), and NetBIOS names (which are used by NetBIOS applications) are discussed.

Host Name Resolution

A host name is an alias assigned to an IP node to identify it as a TCP/IP host. The host name can be up to 255 characters long and can contain alphabetic and numeric characters and the “-” and “.” characters“.” Multiple host names can be assigned to the same host. For Windows Server 2003–based computers, the host name does not have to match the Windows Server 2003 computer name.

Windows Sockets applications, such as Microsoft Internet Explorer, can use one of two values to connect to the destination: the IP address or a host name. When the IP address is specified, name resolution is not needed. When a host name is specified, the host name must be resolved to an IP address before IP-based communication with the desired resource can begin.

Host names most commonly take the form of a domain name with a structure that follows Internet conventions. Name resolution, and domain names work the same whether they are used for IPv4 or IPv6 addresses.

Domain Names

To meet the need for a scaleable, customizable naming scheme for a wide variety of organizations, InterNIC has created and maintains a hierarchical namespace called the Domain Name System (DNS). The DNS naming scheme looks like the directory structure for files on a disk. Usually, you trace a file path from the root directory through subdirectories to its final location and its file name. However, a host name is traced from its final location back through its parent domains up to the root. The unique name of the host, representing its position in the hierarchy, is its Fully Qualified Domain Name (FQDN). The top-level domain namespace with second-level and subdomains is shown in the following figure.

Domain Name System

The domain namespace includes the following categories:

• The root domain, which is indicated by “” (null), represents the root of the namespace.

• Top-level domains, directly below the root, represent types of organizations. InterNIC is responsible for the maintenance of top-level domain names on the Internet. The following table has a partial list of the Internet’s top-level domain names.

Internet Top-Level Domain Names

Domain Name	Meaning
com	Commercial organization
edu	Educational institution
gov	Government institution
mil	Military group
net	Major network support center
org	Organization other than those above
int	International organization
<country/region code>	Each country/region (geographic scheme)

• Second-level domains, below the top-level domains, represent specific organizations within the top-level domains. InterNIC is responsible for maintaining and ensuring uniqueness of second-level domain names on the Internet.

• Subdomains are below the second-level domain. Individual organizations are responsible for the creation and maintenance of subdomains.

For example, for the FQDN websrv.wcoast.reskit.com:

• The trailing period (.) denotes that this is an FQDN with the name relative to the root of the domain namespace. The trailing period is usually not required for FQDNs and if it is missing it is assumed to be present.

• com is the top-level domain, indicating a commercial organization.

• reskit is the second-level domain, indicating the Resource Kit Corporation.

• wcoast is a subdomain of reskit.com indicating the West Coast division of the Resource Kit Corporation.

• websrv is the name of the Web server in the West Coast division.

Domain names are not case-sensitive.

Organizations not connected to the Internet can implement whatever top and second-level domain names they want. However, typical implementations follow InterNIC specifications so that eventual participation in the Internet will not require a renaming process.

Host Name Resolution Using a Hosts File

One common way to resolve a host name to an IP address is to use a locally stored database file that contains IP-address-to-host-name mappings. On most UNIX systems, this file is /etc/hosts. On Windows Server 2003 systems, it is the Hosts file in the %systemroot%\System32\Drivers\Etc directory.

The following is an example of the contents of the Hosts file:

Table of IP addresses and host names
127.0.0.1    localhost
131.107.34.1    router
172.30.45.121    server1.central.reskit.com s1

Within the Hosts file:

• Multiple host names can be assigned to the same IP address. Note that the server at the IP address 172.30.45.121 can be referred to by its FQDN (server1.central.reskit.com) or a nickname (s1). This allows the user at this computer to refer to this server using the nickname s1 instead of typing the entire FQDN.

• Entries can be case sensitive depending on the platform. Entries in the Hosts file for UNIX computers are case-sensitive. Entries in the Hosts file for Windows Server 2003, Windows XP, and Windows 2000–based computers are not case sensitive.

For computers running Windows Server 2003, Windows XP, and Windows 2000, the entries in the Hosts file are loaded into the DNS client resolver cache. When resolving host names, the DNS client resolver cache is always checked.

The advantage of using a Hosts file is that it is customizable for the user. Users can create whatever entries they want, including easy-to-remember nicknames for frequently accessed resources. However, the individual maintenance of the Hosts file does not scale well to storing large numbers of FQDN mappings.

Host Name Resolution Using a DNS Server

To make host name resolution scalable and centrally manageable, IP address mappings for FQDNs are stored on DNS servers. To enable the querying of a DNS server by a host computer, a component called the DNS resolver is enabled and configured with the IP address of the DNS server. The DNS resolver is a built-in component of TCP/IP protocol stacks supplied with most network operating systems, including Windows Server 2003.

When a Windows Sockets application is given an FQDN as the destination location, the application calls a Windows Sockets function to resolve the name to an IP address. The request is passed to the DNS resolver component in the TCP/IP protocol. The DNS resolver packages the FQDN request as a DNS Name Query packet and sends it to the DNS server.

DNS is a distributed naming system. Instead of storing all the records for the entire namespace on each DNS server, each DNS server stores only the records for a specific portion of the namespace. The DNS server is authoritative for the portion of the namespace that corresponds to records stored on that DNS server. In the case of the Internet, hundreds of DNS servers store various portions of the Internet namespace. To facilitate the resolution of any valid domain name by any DNS server, DNS servers are also configured with pointer records to other DNS servers.

The following process outlines what happens when the DNS resolver component on a host sends a DNS query to a DNS server. This process is shown in the following figure and is simplified so that you can gain a basic understanding of the DNS resolution process.

1. The DNS resolver component of the DNS client formats a DNS Name Query Request message containing the FQDN and sends it to the configured DNS server.

2. The DNS server checks the FQDN in the DNS Name Query Request message against locally stored address records. If a record is found, the IP address corresponding to the requested FQDN is sent back to the client.

3. If the FQDN is not found, the DNS server forwards the request to a DNS server that is authoritative for the FQDN.

4. The authoritative DNS server returns the reply, which contains the resolved IP address, back to the original DNS server.

5. The original DNS server sends the IP address mapping information to the client.

Resolving an FQDN by using DNS servers

To obtain the IP address of a server that is authoritative for the FQDN, DNS servers on the Internet go through an iterative process of querying multiple DNS servers until the authoritative server is found. For more information about DNS name-resolution processes, see the DNS Technical Reference.

Combining a Local Database File with DNS

TCP/IP implementations, including Windows Server 2003, allow the use of both a local database file and a DNS server to resolve host names. When a user specifies a host name in a Windows Sockets–based TCP/IP application:

• TCP/IP checks the DNS client resolver cache (loaded with entries from the Hosts file and other previously resolved host names) for a matching name. If a matching name is not found in the local database file, the host name is packaged as a DNS Name Query Request message and sent to the configured DNS server.

Combining methods allows the user to have a local database file for resolving personalized nicknames and to use the globally distributed DNS database to resolve FQDNs.

NetBIOS Name Resolution

NetBIOS name resolution is the process of successfully mapping a NetBIOS name to an IP address. A NetBIOS name is a 16-byte address used to identify a NetBIOS resource on the network. A NetBIOS name is either a unique (exclusive) or group (nonexclusive) name. When a NetBIOS process communicates with a specific process on a specific computer, a unique name is used. When a NetBIOS process communicates with multiple processes on multiple computers, a group name is used.

The NetBIOS name acts as a Session layer application identifier. For example, the NetBIOS session service operates over TCP port 139. All NetBT session requests are addressed to TCP destination port 139. When identifying a NetBIOS application with which to establish a NetBIOS session, the NetBIOS name is used.

An example of a process using a NetBIOS name is the File and Printer Sharing for Microsoft Networks component (the Server service) on a Windows Server 2003–based computer. When you start your computer, the Server service registers a unique NetBIOS name based on your computer’s name. The exact name used by the Server service is the 15-character computer name plus a sixteenth character of 0x20. If the computer name is not 15 characters long, it is padded with spaces up to 15 characters long. Other network services, such as the Workstation or Messenger service, also use the computer name to build their NetBIOS names. The sixteenth character is used to uniquely identify each service.

Note

• The Messenger service refered to here is not Windows Messenger. Windows Messenger is a Microsoft application included in Windows Server 2003 that allows real-time messaging and collaboration.

The Server service on the file server you specify corresponds to a specific NetBIOS name. For example, when you attempt to connect to the computer called CORPSERVER, the NetBIOS name corresponding to the Server service is "CORPSERVER <20>" (note the padding using the space character). Before a file and print sharing connection can be established, a TCP connection must be created. In order for a TCP connection to be established, the NetBIOS name "CORPSERVER <20>" must be resolved to an IP address.

To view the NetBIOS names registered by NetBIOS processes running on a Windows Server 2003 computer, type nbtstat -n at the Windows Server 2003 command prompt.

NetBIOS Node Types

The exact mechanism by which NetBIOS names are resolved to IP addresses depends on the node’s configured NetBIOS Node Type. RFC 1001 defines the NetBIOS Node Types listed in the following table.

NetBIOS Node Types

Node Type	Description
B-node (broadcast)	B-node uses broadcasted NetBIOS name queries for name registration and resolution. B-node has two major problems: (1) In a large internetwork, broadcasts can increase the network load, and (2) Routers typically do not forward broadcasts, so only NetBIOS names on the local network can be resolved.
P-node (peer-peer)	P-node uses a NetBIOS name server (NBNS), such as Windows Internet Name Service (WINS), to resolve NetBIOS names. P-node does not use broadcasts; instead, it queries the name server directly. The most significant problem with P-node is that all computers must be configured with the IP address of the NBNS, and if the NBNS is down, computers are not able to communicate even on the local network.
M-node (mixed)	M-node is a combination of B-node and P-node. By default, an M-node functions as a B-node. If it is unable to resolve a name by broadcast, it uses the NBNS of P-node.
H-node (hybrid)	H-node is a combination of P-node and B-node. By default, an H-node functions as a P-node. If it is unable to resolve a name through the NetBIOS name server, it uses a broadcast to resolve the name.

When NetBT is enabled, Windows Server 2003–based computers are B-node by default and become H-node when configured for a WINS server. Windows Server 2003 also uses a local database file called Lmhosts to resolve remote NetBIOS names.

IPv4 Routing

After the host name or NetBIOS name is resolved to an IP address, the IP packet must be sent by the sending host to the resolved IP address. Routing is the process of forwarding a packet based on the destination IP address. Routing involves both the TCP/IP host and an IP router. A router is a device that forwards the packets from one network to another. Routers are also commonly referred to as gateways. Both the sending host and router need to make a determination about how the packet is forwarded.

To make these determinations, the IP layer consults a routing table stored in memory. Routing table entries are created by default when TCP/IP initializes and additional entries are added either manually by a system administrator or automatically through communication with routers.

Direct and Indirect Delivery

IP packets use at least one of two types of delivery based on whether the final destination is located on a directly attached network. These two types of delivery are known as direct and indirect delivery.

• Direct delivery occurs when the IP node (either the sending node or an IP router) forwards a packet to the final destination on a directly attached network. The IP node encapsulates the IP packet in a frame format for the Network Interface layer (such as Ethernet or Token Ring) addressed to the destination’s MAC address.

• Indirect delivery occurs when the IP node (either the sending node or an IP router) forwards a packet to an intermediate node (an IP router) because the final destination is not on a directly attached network. The IP node encapsulates the IP packet in a frame format for the Network Interface layer (such as Ethernet or Token Ring) addressed to the IP router’s MAC address.

IP routing is a combination of direct and indirect deliveries.

In the following figure, when sending packets to node B, node A performs a direct delivery. When sending packets to node C, node A performs an indirect delivery to Router 1, and Router 1 performs an indirect delivery to Router 2, and then Router 2 performs a direct delivery to node C.

Direct and Indirect Deliveries

IP Routing Table

A routing table is present on all IP nodes. The routing table stores information about IP networks and how they can be reached (either directly or indirectly). Because all IP nodes perform some form of IP routing, routing tables are not exclusive to IP routers. Any node loading the TCP/IP protocol has a routing table. There are a series of default entries according to the configuration of the node and additional entries can be entered either manually through TCP/IP utilities or dynamically through interaction with routers.

When an IP packet is to be forwarded, the routing table is used to determine:

• The next-hop IP address. For a direct delivery, the next-hop IP address is the destination IP address in the IP packet. For an indirect delivery, the next-hop IP address is the IP address of a router.

• The next-hop interface. The next-hop interface identifies the physical or logical interface, such as a network adapter, that is used to forward the packet to either its destination or the next router.

IP Routing Table Entry Types

Entries in the IP routing table contain the following information:

• Network ID. The network ID or destination corresponding to the route. The network ID can identify a specific subnet, be a summarized route, or an IP address for a host route. In the Windows Server 2003 IP routing table, this is the Network Destination column.

• Network mask. The mask that is used to match a destination IP address to the network ID. In the Windows Server 2003 IP routing table, this is the Netmask column.

• Next hop. The IP address of the next hop. In the Windows Server 2003 IP routing table, this is the Gateway column.

• Interface. An indication of which network interface is used to forward the IP packet.

• Metric. A number used to indicate the cost of the route so the best route among possible multiple routes to the same destination can be selected. A common use of the metric is to indicate the number of hops (routers crossed) to the network ID.

Entries in the routing table can be used to store the following types of routes:

• Directly attached network ID. Aroute for network IDs that are directly attached. For directly attached networks, the Next Hop field can be blank or contain the IP address of the interface on that network.

• Remote network ID. A route for network IDs that are not directly attached but are available across other routers. For remote networks, the Next Hop field is the IP address of a local router.

• Host route. A route to a specific IP address. Host routes allow routing to occur on a per-IP address basis. For host routes, the network ID is the IP address of the specified host and the network mask is 255.255.255.255.

• Default route. The default route is designed to be used when a more specific network ID or host route is not found. The default route network ID is 0.0.0.0 with a network mask of 0.0.0.0.

Route Determination Process

To determine which routing table entry is used to find the next-hop address and interface, IP uses the following process:

• For each entry in a routing table, IP performs a bit-wise logical AND operation between the destination IP address and the network mask. It compares the result with the network ID of the entry for a match.

• A list of matching routes is compiled. The route that has the longest match (the route with the largest number of bits that match the destination IP address) is chosen. The longest matching route is the most direct route to the destination IP address. If multiple matching entries are found (for example, multiple routes to the same network ID), the router uses the lowest metric to select the best route. If multiple entries have the longest match and the lowest metric, the router designates one of them as the routing table entry. For Windows Server 2003 TCP/IP, the route chosen corresponds to the route associated with the interface that is first in the network binding order.

The end result of the route-determination process is a single route in the routing table that yields a next-hop IP address and interface. If the route-determination process fails to find a route, IP indicates a routing error. For the sending host, an IP routing error message is sent to the upper layer protocol, such as TCP or UDP. For a router, an ICMP Destination Unreachable–Host Unreachable message is sent to the sending host.

Routing Table for Windows Server 2003

The following table shows the default routing table for a Windows Server 2003–based host (not a router). The host has a single network adapter and has the IP address 157.60.27.90, subnet mask 255.255.240.0, and a default gateway of 157.60.16.1.

Windows Server 2003 Routing Table

Network Destination	Netmask	Gateway	Interface	Metric	Purpose
0.0.0.0	0.0.0.0	157.60.16.1	157.60.27.90	1	Default Route
127.0.0.0	255.0.0.0	127.0.0.1	127.0.0.1	1	Loopback Network
157.60.16.0	255.255.240.0	157.60.27.90	157.60.27.90	1	Directly Attached Network
157.60.27.90	255.255.255.255	127.0.0.1	127.0.0.1	1	Local Host
157.60.255.255	255.255.255.255	157.60.27.90	157.60.27.90	1	Network Broadcast
224.0.0.0	240.0.0.0	157.60.27.90	157.60.27.90	1	Multicast
255.255.255.255	255.255.255.255	157.60.27.90	157.60.27.90	1	Limited Broadcast

Default Route

The entry corresponding to the default gateway configuration is a network destination of 0.0.0.0 with a network mask (netmask) of 0.0.0.0. Any destination IP address that is logically ANDed with 0.0.0.0 results in 0.0.0.0. Therefore, for any IP address, the default route produces a match. If the default route is chosen because no better routes were found, the IP packet is forwarded to the IP address in the Gateway column (the default gateway of 157.60.16.1), by using the interface assigned the IP address in the Interface column.

Loopback Network

The loopback network entry is designed to take any IP address of the form 127.x.y.z and forward it to the special loopback address of 127.0.0.1.

Directly Attached Network

The local network entry corresponds to the directly attached network. IP packets destined for the directly attached network are not forwarded to a router but sent directly to the destination. Note that the Gateway and Interface columns match the IP address of the node. This indicates that the packet is sent from the network adapter corresponding to the node’s IP address.

Local Host

The local host entry is a host route (network mask of 255.255.255.255) corresponding to the IP address of the host. All IP packets sent to the IP address of the host are forwarded to the loopback address.

Network Broadcast

The network broadcast entry is a host route (network mask of 255.255.255.255) corresponding to the all-subnets directed broadcast address (all subnets of class B network ID 157.60.0.0). Packets addressed to the all-subnets directed broadcast are sent from the network adapter corresponding to the node’s IP address.

Multicast

The multicast addresses route is used to send any multicast IP packets from the network adapter corresponding to the node’s IP address.

Limited Broadcast

The limited broadcast address is a host route (network mask of 255.255.255.255). Packets addressed to the limited broadcast are sent from the network adapter corresponding to the node’s IP address.

Viewing the IP Routing Table

To view the IP routing table on a Windows Server 2003-based computer, type route print at a Windows Server 2003 command prompt.

When determining the next-hop IP address and interface from a route in the routing table:

• If the gateway address is the same as the interface address, the next-hop IP address is set to the destination IP address of the IP packet.

• If the gateway address is not the same as the interface address, the next-hop IP address is set to the gateway address.

For example, when traffic is sent to 157.60.16.48, the most specific matching route is the route for the directly attached network (157.60.16.0/20). The next-hop IP address is set to the destination IP address (157.60.16.48) and the interface is the network adapter that has been assigned the IP address 157.60.27.90.

When sending traffic to 192.168.0.79, the most specific matching route is the default route (0.0.0.0/0). The next-hop IP address is set to the gateway address (157.60.16.1) and the interface is the network adapter that has been assigned the IP address 157.60.27.90.

Maintenance of Routing Table Entries

For IP routing to occur efficiently between routers in the IP internetwork, routers must be configured with remote network IDs or a default route. On large IP internetworks, one of the challenges faced by network administrators is how to maintain the routing tables on their IP routers so that IP traffic flow is traveling the best path and is fault tolerant.

There are two methods of maintaining routing table entries on IP routers.

Manual

Static IP routers have routing tables that do not change unless manually changed by a network administrator.

Static routing relies on the manual administration of the routing table. Remote network IDs are not discovered by static routers and must be manually configured. Static routers are not fault-tolerant. If a static router goes down, neighboring routers do not sense the fault and inform other routers.

Automatic

Dynamic IP routers have routing tables that change automatically based on the exchange of routing information with other routers.

Dynamic routing employs the use of routing protocols, such as Routing Information Protocol (RIP) and Open Shortest Path First (OSPF), to dynamically update the routing table through the exchange of routing information between routers. Remote network IDs are discovered by dynamic routers and automatically entered into the routing table. Dynamic routers are fault-tolerant. If a dynamic router goes down, the fault is detected by neighboring routers, which send the changed routing information to the other routers in the internetwork.

Physical Address Resolution

Based on the destination IP address and the route determination process, IP determines the next-hop IP address and interface. IP then sends the IP packet, the next-hop IP address, and the interface to ARP.

If the next-hop IP address is the same as the destination IP address, then ARP performs a direct delivery. In a direct delivery, the MAC address corresponding to the destination IP address must be resolved.

If the next-hop IP address is not the same as the destination IP address, then ARP performs an indirect delivery. The next-hop IP address is the IP address of a router between the current IP node and the final destination. In an indirect delivery, the MAC address corresponding to the IP address of the router must be resolved.

To resolve a next-hop IP address to its MAC address, ARP uses broadcast traffic on shared access networking media (such as Ethernet or Token Ring) to send out a broadcasted ARP Request frame. An ARP Reply, containing the MAC address corresponding to the requested next-hop IP address, is sent back to the sender of the ARP Request.

ARP Cache

To keep the number of broadcasted ARP Request frames to a minimum, many TCP/IP protocol stacks incorporate an ARP cache, which is a table of recently resolved IP addresses and their corresponding MAC addresses. TCP/IP checks the ARP cache before sending an ARP Request frame. Each interface has its own ARP cache.

Depending on the vendor implementation, the ARP cache can have the following qualities:

• ARP cache entries can be dynamic (based on ARP Replies) or static. Static ARP entries are permanent and are manually added by using a TCP/IP utility such as the ARP tool provided with Windows Server 2003. Static ARP cache entries are used to prevent ARP Requests for commonly used local IP addresses, such as routers and servers. The problem with static ARP entries is that they have to be manually updated when network interface equipment changes.

• Dynamic ARP cache entries have a time-out value associated with them to remove entries in the cache after a specified period of time. Dynamic ARP cache entries for Windows Server 2003 TCP/IP are given a maximum time of 10 minutes before being removed.

To view the ARP cache on a Windows Server 2003–based computer, type arp -a at a Windows Server 2003 command prompt.

ARP Process

IP sends ARP the IP packet, the next-hop IP address, and the next-hop interface. Whether performing a direct or indirect delivery, ARP carries out the following process, as shown in the following figure.

ARP process

1. Based on the next-hop address and interface, ARP consults the appropriate ARP cache for an entry for the next-hop IP address. If an entry is found, ARP skips to step 6.

2. If an entry is not found, ARP builds an ARP Request frame containing the MAC address of the interface sending the ARP Request, the IP address of the interface sending the ARP Request, and the next-hop IP address. ARP then broadcasts the ARP Request using the appropriate interface.

3. All hosts receive the broadcasted frame and the ARP Request is processed. If the receiving host’s IP address matches the requested IP address (the next-hop IP address), its ARP cache is updated with the address mapping of the sender of the ARP Request.

4. If the receiving host’s IP address does not match the requested IP address, the ARP Request is silently discarded.

5. The receiving host formulates an ARP Reply containing the requested MAC address and sends it directly to the sender of the ARP Request.

6. When the ARP Reply is received by the sender of the ARP Request, it updates its ARP cache with the address mapping.

7. The ARP Request host and the ARP Reply host have each other’s address mappings in their ARP caches.

8. ARP sends the IP packet to the next-hop node by addressing it to the resolved MAC address.

End-to-End Delivery

The IP routing processes for all nodes involved in the delivery of an IP packet include the sending host, the intermediate routers, and the destination host.

IP on the Sending Host

When a host sends a packet, the packet is transmitted from an upper layer protocol (TCP, UDP, or ICMP) to IP, and then IP on the sending host does the following:

1. Sets the Time-to-Live (TTL) value to either a default or application-specified value.

2. Checks its routing table for the best route to the destination IP address.

   If no route is found, IP sends a routing error message to the upper layer protocol (TCP, UDP, or ICMP).

3. Determines the next-hop IP address and the interface, based on the most specific matching route.

4. Sends the packet, the next-hop IP address, and the next-hop interface to Address Resolution Protocol (ARP), and then ARP resolves the next-hop IP address to its media access control (MAC) address and forwards the packet.

IP on the Router

When a packet is received at a router, the packet is passed to IP, and IP on the router does the following:

1. Verifies the IP header checksum.

   If the IP header checksum fails, the IP packet is discarded without notification to the user. This is known as a silent discard.

2. Verifies whether the destination IP address in the IP packet corresponds to an IP address assigned to a router interface.

   If so, the router processes the IP packet as the destination host (see step 3 in the following “IP on the Destination Host” section).

3. If the destination IP address is not the router, IP decrements the Time-to-Live (TTL).

   If the TTL is 0, the router discards the packet and sends an ICMP Time Expired–TTL Expired in Transit message to the sender.

4. If the TTL is 1 or greater, IP updates the TTL field and calculates a new IP header checksum.

5. IP checks its routing table for the best route to the destination IP address in the IP packet.

   If no route is found, the router discards the packet and sends an ICMP Destination Unreachable–Host Unreachable message to the sender.

6. Based on the best route found, IP determines the next-hop IP address and interface.

7. IP sends the packet, the next-hop IP address, and the interface to ARP, and then ARP forwards the packet to the appropriate MAC address.

This entire process is repeated at each router in the path between the source and destination host.

IP on the Destination Host

When a packet is received at the destination host, it is passed up to IP, and IP on the destination host does the following:

1. Verifies the IP header checksum.

   If the IP header checksum fails, the IP packet is silently discarded.

2. Verifies that the destination IP address in the IP packet corresponds to an IP address assigned to the host.

   If the destination IP address is not assigned to the host, the IP packet is silently discarded.

3. Passes the IP packet without the IP header to the appropriate upper-level protocol, based on the IP protocol field.

   If the protocol does not exist, ICMP sends a Destination Unreachable–Protocol Unreachable message back to the sender.

4. For TCP and UDP packets, IP checks the destination port and processes the TCP segment or UDP header.

   If no application exists for the UDP port number, ICMP sends a Destination Unreachable–Port Unreachable message back to the sender. If no application exists for the TCP port number, TCP sends a Connection Reset segment back to the sender.

Related Information

• For more information about RFCs, see the IETF RFC Database.