DNS domain names

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

DNS domain names

Domain Name System (DNS) was originally defined in Request for Comments (RFCs) 1034 and 1035. These documents specify elements common to all implementations of DNS-related software, including:

• A DNS domain namespace, which specifies a structured hierarchy of domains used to organize names.

• Resource records, which map DNS domain names to a specific type of resource information for use when the name is registered or resolved in the namespace.

• DNS servers, which store and answer name queries for resource records.

• DNS clients, also known as resolvers, which query servers to look up and resolve names to a type of resource record specified in the query.

For more information about Requests for Comments (RFCs), see DNS RFCs.

Understanding the DNS domain namespace

The DNS domain namespace, as shown in the following figure, is based on the concept of a tree of named domains. Each level of the tree can represent either a branch or a leaf of the tree. A branch is a level where more than one name is used to identify a collection of named resources. A leaf represents a single name used once at that level to indicate a specific resource.

The previous figure shows how Microsoft is assigned authority by the Internet root servers for its own part of the DNS domain namespace tree on the Internet. DNS clients and servers use queries as the fundamental method of resolving names in the tree to specific types of resource information. This information is provided by DNS servers in query responses to DNS clients, who then extract the information and pass it to a requesting program for resolving the queried name.

In the process of resolving a name, keep in mind that DNS servers often function as DNS clients, querying other servers in order to fully resolve a queried name. For more information, see How DNS query works.

How the DNS domain namespace is organized

Any DNS domain name used in the tree is technically a domain. Most DNS discussions, however, identify names in one of five ways, based on the level and the way a name is commonly used. For example, the DNS domain name registered to Microsoft (microsoft.com.) is known as a second-level domain. This is because the name has two parts (known as labels) that indicate it is located two levels below the root or top of the tree. Most DNS domain names have two or more labels, each of which indicates a new level in the tree. Periods are used in names to separate labels.

In addition to second-level domains, other terms used to describe DNS domain names by their function in the namespace are described in the following table.

Name type	Description	Example
The domain root	This is the top of the tree, representing an unnamed level; it is sometimes shown as two empty quotation marks (""), indicating a null value. When used in a DNS domain name, it is stated by a trailing period (.) to designate that the name is located at the root or highest level of the domain hierarchy. In this instance, the DNS domain name is considered to be complete and points to an exact location in the tree of names. Names stated this way are called fully qualified domain names (FQDNs).	A single period (.) or a period used at the end of a name, such as "example.microsoft.com.".
Top-level domain	A name of two or three letters used to indicate a country/region or the type of organization using a name. For more information, see Top-level domains.	".com", which indicates a name registered to a business for commercial use on the Internet.
Second-level domain	Variable-length names registered to an individual or organization for use on the Internet. These names are always based upon an appropriate top-level domain, depending on the type of organization or geographic location where a name is used.	"microsoft.com.", which is the second-level domain name registered to Microsoft by the Internet DNS domain name registrar.
Subdomain	Additional names that an organization can create that are derived from the registered second-level domain name. These include names added to grow the DNS tree of names in an organization and divide it into departments or geographic locations.	"example.microsoft.com.", which is a fictitious subdomain assigned by Microsoft for use in documentation example names.
Host or resource name	Names that represent a leaf in the DNS tree of names and identify a specific resource. Typically, the leftmost label of a DNS domain name identifies a specific computer on the network. For example, if a name at this level is used in a host (A) RR, it is used to look up the IP address of computer based on its host name.	"host-a.example.microsoft.com.", where the first label ("host-a") is the DNS host name for a specific computer on the network.

Interpreting a DNS domain name

DNS has a method of noting and interpreting the fully qualified path to a DNS domain name similar to the way full paths to files or directories are noted or displayed at a command prompt.

For example, a directory tree path helps point to the exact location of a file stored on your computer. For Windows computers, the back slash (\) indicates each new directory that leads to the exact location of a file. For DNS, the equivalent is a period (.) indicating each new domain level used in a name.

For example, for a file called Services, the full path to this file as displayed at a Windows command prompt would be:

C:\Windows\System32\Drivers\Etc\Services

To interpret the full path of the file, the name is read in a left-to-right direction from the highest or most general piece of information (drive C, the drive where the file is stored) to its most specific information, the file name "Services". This example shows five separate levels of hierarchy that lead toward the location of the Services file on drive C:

1. The root folder for drive C (C:\).

2. The system root folder where Windows is installed (Windows).

3. A system folder where system components are stored (System32).

4. A subfolder where system device drivers are stored (Drivers).

5. A subfolder where miscellaneous files used by system and network device drivers are stored (Etc).

For DNS, an example of a domain name with multiple levels is the following, a fully qualified domain name (FQDN):

host-a.example.microsoft.com.

Unlike the file name example, a DNS FQDN, when read from left to right, moves from its most specific information (the DNS name for a computer called "host-a") to its highest or most general piece of information (the trailing period (.) that indicates the root of the DNS name tree). This example shows the four separate DNS domain levels that lead away from the specific host location of "host-a":

1. The "example" domain, which corresponds to a subdomain where the computer name "host-a" is registered for use.

2. The "microsoft" domain, which corresponds to the parent domain that roots the "example" subdomain.

3. The "com" domain, which corresponds to the top-level domain designated for use by business or commercial organizations that roots the "microsoft" domain.

4. The trailing period (.), which is a standard separator character used to qualify the full DNS domain name to the root level of the DNS namespace tree.

Background on DNS and the Internet

Domain Name System (DNS) was developed because of the need to provide a name-to-address mapping service for computers on the Internet. Before DNS was introduced in 1987, the practice of mapping friendly computer names to IP addresses was done mainly through the use of a shared static file, known as a Hosts file.

Originally, the Internet was small enough to use one centrally administered file that was published and downloaded using FTP for Internet-connected sites. Periodically, each Internet site would update its copy of the Hosts file, and updated versions of the Hosts file were posted to reflect network changes.

As the number of computers on the Internet grew, it became unworkable to have one centralized authority managing a single Hosts file for all Internet hosts. The file became increasingly larger, which made it harder to maintain and distribute to all sites in a current and updated form.

The DNS standard was developed to provide an alternative to Hosts files. RFCs 1034 and 1035 specify most of the core protocols and have been added to and updated by additional RFCs submitted to the Internet Engineering Task Force (IETF). The IETF reviews and approves new drafts continuously, so the standards for DNS evolve and change as needed.

For more information, see DNS RFCs.

Notes

• DNS domain names are required to be unique at each level, but individual name labels can be reused in other domains. For example, the name "mailserver" can be used one time only in both the example.microsoft.com and microsoft.com domains.

• For more information on the use of labels, FQDNs, and other DNS host naming requirements, see Namespace planning for DNS.

• Hosts files are supported as a local static file method of mapping DNS domain names for host computers to their IP addresses. When the DNS Client service is started, it preloads any mapped entries added to this file into the local DNS names caches.

• The Hosts file is provided in the systemroot\System32\Drivers\Etc folder. To view or modify this file, you can use Notepad or another text-based editor. For more information about creating or editing the Hosts file, refer to the comments and examples in the file.