DNS organizes groups of computers into domains. These domains are organized into a hierarchical structure that can be defined on an Internet-wide basis for public networks or on an enterprise-wide basis for private networks (also known as extranets and intranets, respectively). The various levels within the hierarchy identify individual computers, organizational domains, and top-level domains. For the fully qualified host name omega.microsoft.com, omega represents the host name for an individual computer, microsoft is the organizational domain, and com is the top-level domain.
Top-level domains are at the root of the DNS hierarchy and are also called root domains . These domains are organized geographically, by organization type, and by function. Typical corporate domains, such as microsoft.com, are also referred to as parent domains because they’re the parents of an organizational structure. You can divide parent domains into subdomains you can use for groups or departments within your organization.
Subdomains are often referred to as child domains. For example, the fully qualified domain name (FQDN) for a computer within a human resources group could be designated as jacob.hr.microsoft.com. Here, jacob is the host name, hr is the child domain, and microsoft.com is the parent domain.