Section 3.4: Domain Name Space

The DNS database hierarchical naming scheme called a domain name space. Each node in the name space hierarchy represents a partition of the DNS database. These nodes are referred to as domains, each of which must have a name as the DNS database is indexed by name. When you add domains to the hierarchy, the name of the parent domain is appended to the domain, which becomes a child domain or a subdomain. Consequently, a domain's name identifies its position in the hierarchy. Thus, the domain name identifies the studyguides domain as a subdomain of the domain and testking as a subdomain of the com domain.

Note: The term domain, in the context of DNS, is not related to domain as used in the Microsoft Windows Server 2003 directory services. A Windows Server 2003 domain is a grouping of computers and devices that are administered as a unit.

The hierarchical structure of the domain name space consists of a root domain, top-level domains, second-level domains, any subdomains, and host names.

• The Root Domain is at the top of the hierarchy and is represented as a period (.).

• Top-Level Domains are two or three-character name codes. Top-level domains are organized by organization type or geographic location. Table 3.1 provides some examples of top-level domain names.

Table 3.1: Top-Level Domains

Top-level Domain



Government organizations


Commercial organizations


Educational institutions


Noncommercial organizations


Country code of China

Top-level domains can contain second-level domains and host names.

• Second-Level Domains are registered to individuals and organizations for use on the Internet. A second-level name has two name parts: a top-level name and a unique second-level name. An example is testking. com.

• Subdomains are created when organizations extend their DNS tree to represent departments, divisions, or other geographic locations. Subdomains have three name parts: a top-level name, a unique second-level name, and a unique name representing the department or location. An example is studyguides. testking. com.

• Host names are the names of specific computers on the Internet or in a private network. 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. is an FQDN and includes the end period. DNS uses a host's FQDN to resolve a name to an IP address. The host name does not have to be the same as the computer name.