Multihomed DHCP servers

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

Multihomed DHCP servers

A multihomed DHCP server is a computer running a Windows Server 2003 operating system that uses the DHCP service for more than a single network connection. For a server computer to be multihomed, each network connection must attach the computer to more than a single physical network. This requires that additional hardware (in the form of multiple installed network adapters) be used on the computer.

A computer running a Windows Server 2003 operating system can perform as a multihomed DHCP server. For multihomed servers, the DHCP service binds to the first IP address statically configured for each network connection in use.

By default, the service bindings depend on whether the first network connection is configured dynamically or statically for TCP/IP. Based on the method of configuration it uses, reflected by its current settings in Internet Protocol (TCP/IP) properties, the DHCP Server service performs default service bindings as follows:

• If the first network connection uses a manually specified IP address, the connection is enabled in server bindings. For this to occur, a value for IP address must be configured and the Use the following IP address option selected in Internet Protocol (TCP/IP) properties. In this mode, the DHCP server listens for and provides service to DHCP clients.

• If the first network connection uses an IP address configured dynamically, the connection is disabled in server bindings. This occurs when the Obtain an IP address automatically option is selected in Internet Protocol (TCP/IP) properties. For computers running Windows Server 2003 operating systems, this is the default setting. In this mode, the DHCP server does not listen for and provide service to DHCP clients until a static IP address is configured.

• The DHCP server will bind to the first static IP address configured on each adapter.

Notes

• By design, DHCP server bindings are enabled and disabled on a per-connection, not per-address basis. All bindings are based on the first configured IP address for each connection appearing in the Network Connections folder. If additional static IP addresses (for example, as set in Advanced TCP/IP properties) are configured for the applicable connection, these addresses are never used by DHCP servers running Windows Server 2003 and are inconsequential for server bindings.

• For more information, see About Network Connections.

• DHCP servers running Windows Server 2003 never bind to any of the NDISWAN or DHCP-enabled interfaces used on the server. These interfaces are not displayed in the DHCP console under the current server bindings list because they are never used for DHCP service. Only additional network connections that have a primary static IP address configured can appear in the server bindings list (or be selectively enabled or disabled there).

• For more information, see Selectively set DHCP server bindings for network connections.

Example: Configuring a multihomed DHCP server

The following illustration is an example of a multihomed DHCP server with three network adapters installed. Each adapter is configured to lease addresses on separate physical subnets.

Note that in the graphic, each network adapter is configured statically with a single IP address. In this network example, three Class C ranges of IP addresses are used:

Subnet	IP Address Range
Subnet A	192.168.0.1 - 192.168.0.254
Subnet B	192.168.1.1 - 192.168.1.254
Subnet C	192.168.2.1 - 192.168.2.254

Each of the three adapters in this example connects the server to three different physical subnets (Subnets A, B, and C). To achieve the intended result of having DHCP provide leased configuration service to all clients in each of the respective subnets, two configuration details are essential and must be verified during deployment plans:

1. The server must use a statically configured IP address on the same subnet as the IP address range it services.

2. The server must have each of its valid subnet IP addresses excluded from the scope used to offer leases to clients.

The default subnet mask value (255.255.255.0) is used for all three subnets, providing 254 IP addresses available per subnet for lease to DHCP clients.

To provide a small pool of ten IP addresses available for statically configured devices such as the DHCP server, printers, and other devices, an exclusion range is created in the scope for each subnet:

Subnet	Exclusion Range
Subnet A	192.168.0.1 - 192.168.0.10
Subnet B	192.168.1.1 - 192.168.1.10
Subnet C	192.168.2.1 - 192.168.2.10

Exclusion ranges prevent the DHCP server from assigning IP addresses to DHCP clients, so all of the IP addresses contained in the exclusion ranges will remain available for static configurations even after the DHCP server is started and begins servicing clients.

Addresses that are not excluded remain available for assignment to DHCP clients. These ranges of IP addresses form each subnet's address pool:

Subnet	Address Pool
Subnet A	192.168.0.11 - 192.168.0.254
Subnet B	192.168.1.11 - 192.168.1.254
Subnet C	192.168.2.11 - 192.168.2.254

Note

• For more information on IP subnetting, see Subnet masks or the Microsoft Windows Server 2003 Resource Kit.