Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 229
Domain Structure

DNS is hierarchical in structure, as shown previously. A domain is a subtree of the domain name space. The advantage of this structure is that at the bottom, the network administrator can assign the names.

From the root, the assigned top-level domains (TLD) are as follows:

Now let’s look at the generalized format for a domain name.

Going down the tree, we can pick out a domain name, such as research.Naugle.com. This would signify the Research department (which is a subdomain of domain Naugle.com) at Naugle Enterprises, which is defined as a commercial entity of the Internet. Naugle.com can be a node in the domain acting as a name server, or there may be different name servers for Naugle.com.

A user at workstation 148.1.1.2 types in the TELNET command and the domain name of host1.research.Naugle.com. This workstation must have the domain name resolver installed on it. This program would send out the translation request to a domain name server to resolve the hostname-to-IP address. If the hostname is found, the domain name server would return the IP address to the workstation. If the name is not found, the server may search for the name elsewhere and return the information to the requesting workstation, or return the address of a name server that the workstation (if able) can query to get more information. More on that in a moment. A domain contains all hosts whose domain names are within a certain domain. A domain name is a sequence of labels separated by dots. A domain is a subdomain of another domain if it is contained within that domain. This relationship can be tested by seeing if the subdomain’s name ends with the containing domain’s name. For example, research.Naugle.com is a subdomain of Naugle.com. Naugle.com is a subdomain of .com, and “ “ (root).

Domain Structure

There are special servers on the Internet that provide guidance to all name servers. These are known as root name servers and, as of this writing, there are nine of them. They do not contain all information about every host on the Internet, but they do provide direction as to where domains are located (the IP address of the name server for the uppermost domain a server is requesting). The root name server is the starting point to find any domain on the Internet. If access to the root servers ceased, transmission over the Internet would eventually come to a halt.