Many networks organize their hosts and routers into a hierarchy of administrative domains. If you are using the NIS or DNS name 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 the InterNIC. If you plan to use DNS, you also need to register your domain name with the InterNIC.
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.
.com – Commercial companies (international in scope)
.edu – Educational institutions (international in scope)
.gov – U.S. government agencies
.fr – France
You select the name that identifies your organization, with the provision that the name must be unique.