Sendmail, 3rd Edition

Overview

DNS stands for Domain Name System. A domain is any logical or physical collection of related hosts or sites. A naming system is best visualized as an inverted tree of information that corresponds to fully qualified hostnames (see Figure 9-1).

Figure 9-1. Domain names form a tree of information

The parts of a fully qualified name are separated from one another with dots. For example:

here.uofa.edu

This name describes the machine here that is part of the uofa subdomain of the edu top-level domain. In Figure 9-1 the dot at the top is the “root” of the tree. It is implied but never^[1] included in fully qualified domain names:

here.uofa.edu.
             ↑implied

The root corresponds to (is served by) actual machines.^[2] Each has knowledge of all the top-level domains (such as gov, com, biz, uk, au, etc.) and the server machines for those domains. Each of the top-level domain’s servers knows of one or more machines with knowledge of the next level below. For example, the server for edu “knows” about the subdomains uofa, uofb, and uofc but might not know about anything below those subdomains, nor about the other domains next to itself such as com.^[3]

A knowledgeable machine, one that can look up or distribute information about its domain and subdomains, is called a name server. Each little black square in Figure 9-1 represents a name server for a portion of a domain. Each is required to have ...