Many networks organize their hosts and routers into a hierarchy of administrative domains. If you are going to use NIS, NIS+, or the DNS name services, you must select a domain name for your organization that is unique worldwide. To ensure that your domain name is unique, you should register it with the InterNIC. This is especially important if you plan to use DNS.
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 could be located below the domain of the parent company. If it 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
The name that identifies your organization is one that you select, with the provision that it is unique.