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.

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:

• Register your DNS domain name with the an appropriate Internet governing body.

• Obtain a network IP address from that governing body.

There are two ways to accomplish this:

• You can communicate directly with the appropriate Internet governing body or their agent. In the United States, InterNIC is the company that currently handles network address and domain registration matters.

• You can contract with an Internet Service Provider (ISP) to assist you. ISPs provide a wide range of services from consulting to actually hosting your Internet presence.

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 ajax.com.

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

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

The ajax.com subdomains now have the following names.

sales.ajax.com
 test.ajax.com
 manf.ajax.com

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

Boss.manf.ajax.com
 quota.Sales.ajax.com

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 Ajax.com domain and delegates authority over the names in that domain. The Ajax.com domain, in turn, assigns names to the hosts in its domain and approves the formation of the Sales.Ajax.com, Test.Ajax.com, and Manf.Ajax.com 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:

ajax.com. 
sales.ajax.com 
test.ajax.com. 
manf.ajax.com

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