Many networks organize their hosts and routers into a hierarchy of administrative domains. If you are using the Network Information Service (NIS) or the Domain Name System (DNS) naming service, you must select a domain name for your organization that is unique worldwide. To ensure that your domain name is unique, you should register the domain name with InterNIC. A unique domain name is required if you plan to allow other sites on the Internet to locate your systems through DNS.
A domain name that is located below another domain is often referred to as a sub-domain. The domain name structure is hierarchical. A new domain typically is located below an existing, related domain. For example, the domain name for a subsidiary company can be located below the domain of the parent company. If the domain name has no other relationship, an organization can place its domain name directly under one of the existing top-level domains such as .com, .org, .edu, .gov, and so forth.