13-27Lesson 2 Understanding the Domain Name SystemRoot domain“ . ”Top-level domainsedu com gov org auexpediamicrosoft congressSecond-level domainssalessales.microsoft.com.Computer1.sales.microsoft.com. Computer1
F13us09
Figure 13-8 The domain namespace is hierarchical in structure.
Root Domain
At the top of the DNS hierarchy, there is a single domain called the root domain, which
is represented by a single period.
Top-Level Domains
Top-level domains are two-, three-, or four-character name codes. Top-level domains
are grouped by organization type or geographic location. Top-level domains are con-
trolled by the Internet Architecture Board (IAB), an Internet authority controlling the
assignment of domain names, among other things. Table 13-4 provides some examples
of top-level domain names.
Table 13-4 Top-Level Domains
Top-Level Domain Descriptiongov Government organizations
com Commercial organizations
edu Educational institutions
org Noncommercial organizations
au Country code of Australia
Top-level domains can contain second-level domains and host names.
Second-Level Domains
Anyone can register a second-level domain name. Second-level domain names are reg-
istered to individuals and organizations by a number of different domain registry com-
panies. A second-level name has two name parts: a top-level name and a unique
second-level name. Table 13-5 provides some examples of second-level domains. After
registering a second-level domain name, you can create as many subdomains of that
domain name as you want. For example, if you registered the domain name con-
toso.com, you could create subdomains such as north.contoso.com, south.con-
toso.com, and so on.
Table 13-5 Second-Level Domains
Second-Level Domain Descriptioned.gov United States Department of Education
Microsoft.com Microsoft Corporation
Stanford.edu Stanford University
w3.org World Wide Web Consortium
pm.gov.au Prime Minister of Australia
Host Names
Host names refer to specific computers on the Internet or a private network. For exam-
ple, in Figure 13-8, Computer1 is a host name. A host name is the leftmost portion of
a fully qualified domain name (FQDN), which describes the exact position of a host
within the domain hierarchy. In Figure 13-8, Computer1.sales.microsoft.com. (includ-
ing the end period, which represents the root domain) is an FQDN.
DNS uses a host’s FQDN to resolve a name to an IP address.
Note The host name does not have to be the same as the computer name. By default, TCP/IP setup uses the computer name for the host name, replacing illegal characters, such as the underscore (_), with a hyphen (-).On the CD At this point, you should view the multimedia presentation “Role of DNS,” which is available in the Multimedia folder on the CD-ROM accompanying this book. This presenta-tion provides valuable insight into how DNS works on a network.
