Solaris Naming Administration Guide

Introducing the DNS Namespace

The entire collection of DNS administrative domains throughout the world are organized in a hierarchy called the DNS namespace. This section shows how the namespace organization affects both local domains and the Internet.

DNS Namespace Hierarchy

Like the UNIX file system, DNS domains are organized as a set of descending branches similar to the roots of a tree. Each branch is a domain, each subbranch is a subdomain. The terms domain and subdomain are relative. A given domain is a subdomain relative to those domains above it in the hierarchy, and a parent domain to the subdomains below it.

Figure 28-3 Domains and Subdomains


For example, in Figure 28-3, com is a parent domain to the Acme, Ajax, and AAA domains. Or you could just as easily say that those are subdomains relative to the com domain. In its turn, the Ajax domain is a parent to four subdomains (Sales, Manf, QA, and Corp).

A domain contains one parent (or top) domain plus the associated subdomains if any. Domains are named up the tree starting with the lowest (deepest) subdomain and ending with the root domain. For example, Mktg.Corp.Ajax.Com. from Figure 28-3.

DNS Hierarchy in a Local Domain

If your company is large enough, it may support a number of domains,organized into a local namespace. Figure 28-4 shows a domain hierarchy that might be in place in a single company. The top-level, or "root" domain for the organization is, which has three sub-domains,,, and

Figure 28-4 Hierarchy of DNS Domains in a Single Organization


DNS clients request service only from the servers that support their domain. If the domain's server does not have the information the client needs, it forwards the request to its parent server, which is the server in the next-higher domain in the hierarchy. If the request reaches the top-level server, the top-level server determines whether the domain is valid. If it is not valid, the server returns a "not found" type message to the client. If the domain is valid, the server routes the request down to the server that supports that domain.

DNS Hierarchy and the Internet

The domain hierarchy shown in Figure 28-4 is, conceptually, a "leaf" of the huge DNS namespace supported on the global Internet.

The DNS namespace for the Internet is organized hierarchically as shown in Figure 28-5. It consists of the root directory, represented as a dot (.) and two top level domain hierarchies, one organizational and one geographical. Note that the com domain introduced in Figure 28-3 is one of a number of top-level organizational domains in existence on the Internet.

Figure 28-5 Hierarchy of Internet Domains


At the present time, the organizational hierarchy divides its namespace into the top-level domains listed shown in Table 28-1. It is probable that additional top-level organizational domains will be added in the future.

Table 28-1 Internet Organizational Domains




Commercial organizations  


Educational institutions 


Government institutions 


Military groups 


Major network support centers 


Nonprofit organizations and others 


International organizations 

The geographic hierarchy assigns each country in the world a two- or three-letter identifier and provides official names for the geographic regions within each country. For example, domains in Britain are subdomains of the uk top-level domain, Japanese domains are subdomains of jp, and so on.

Joining the Internet

The Internet root domain, top-level domains (organizational and geographical) are maintained by the various Internet governing bodies. People with networks of any size can "join" the Internet by registering their domain name in either the organizational or the geographical hierarchy.

Every DNS domain must have a domain name. If your site wants to use DNS for name service without connecting to the Internet, you can use any name your organization wants for its your domains and subdomains, if applicable. However, if your site plans wants to join the Internet, it must register its domain name with the Internet governing bodies.

To join the Internet, you have to:

There are two ways to accomplish this:

Domain Names

Domain names indicate a domain's position in the overall DNS namespace, much as path names indicate a file's position in the UNIX file system. After your local domain is registered, its name is prepended to the name of the Internet hierarchy to which it belongs. For example, the ajax domain shown in Figure 28-4 has been registered as part of the Internet com hierarchy. Therefore, its Internet domain name becomes

Figure 28-6 shows the position of the domain in the DNS namespace on the Internet.

Figure 28-6 Ajax Domain's Position in the DNS Namespace


The subdomains now have the following names.

DNS does not require domain names to be capitalized, though they may be. Here are some examples of machines and domain names:

The Internet organization regulates administration of its domains by granting each domain authority over the names of its hosts and by expecting each domain to delegate authority to the levels below it. Thus, the com domain has authority over the names of the hosts in its domain. It also authorizes the formation of the domain and delegates authority over the names in that domain. The domain, in turn, assigns names to the hosts in its domain and approves the formation of the,, and domains.

Fully-Qualified Domain Names

A domain name is said to be fully-qualified when it includes the names of every DNS domain from the local domain on up to ".", the DNS root domain. Conceptually, the fully-qualified domain name indicates the path to the root, as does the absolute path name of a UNIX file. However, fully-qualified domain names are read from lowest, on the left, to highest, on the right. Therefore, a fully-qualified domain name has the syntax:


The fully qualified domain names for the ajax domain and its subdomains are:

Note the dot at the furthest right position of the name.