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Lesson 10 Domain Name Service Structure
ObjectiveDescribe Internet Domain Names

Hierarchical tree structure

Domain names follow a strict naming convention with a three-level hierarchy: root level, top level, and second level. Each part of the name is separated with a period, the "dot" you hear when someone says a Web address out loud. The figure below illustrates the domain name hierarchy.
Domain Name Hierarchy
Domain Name Hierarchy

Root-level domains

The root-level domain is the starting point in the hierarchy.

Top-level domains

There are two types of top-level domains: original and country. The table below lists the original domain names and the types of organizations to which they are assigned.
Original domain names
Country-level domains are called country code top-level domains (ccTLDs). They correspond to a country, territory, or other geographic location. Examples are us (United States), uk (United Kingdom), de (Germany), and jp (Japan). The list of valid domain names is constantly being revised.
Refer to, for more information about top-level domain names.


In addition to a country-level domain, the us domain is further divided into subdomains, with one subdomain for each state and one for Washington, D.C. The state subdomains are further divided into cities, counties, or other regional groupings. For example, is Cleveland, OH; is San Francisco, CA. While most private domains don't utilize this tedious naming convention, many government agencies do. The trend, however, is toward the simpler dot-origin name method for these government Web sites.

Second-level domains

Second-level domain names can contain both hosts and other domains called subdomains. Host names are added to the beginning of the domain name. For example, in, the name of the host is
Subdomain names may also be added to the domain name; for example,
Descriptions of the domains and their associated levels are illustrated in the MouseOver below:
In the next lesson, you will learn about the functions, components, and types of URLs.