As previously described, the second part of a basic URL used to connect to a specific computer includes the DNS host name, the name of the enterprise or organization domain in which the intranet server is located, and the name of the top-level Internet domain in which the enterprise is registered.
DNS is based on a conceptual hierarchical structure called the domain name space. The root or topmost level of the domain name space is managed by the Internet Network Information Center (InterNIC). In the United States, there are seven domain name servers providing name resolution services at the root level. When you install Microsoft DNS server, the Internet Protocol (IP) addresses of these root name servers are made available in a file named cache.dns.
Under the top-level domains are additional groupings referred to as second-level domains. Second-level domains can also be divided into other, lower levels with additional domains, which themselves can be subdivided into more lower-level domains, and so on.
One of the top-level domains of the Internet is the .com domain, a commercial domain used mainly within the United States. Contained within the .com domain are second-level domains that contain the enterprises registered with InterNIC as part of the .com domain. The names of each of these second-level domains identify the enterprise connected with that particular domain; for example, the domain name microsoft.com identifies the domain containing all computers that exist within Microsoft Corporation.
Individual computers exist within a domain. Each computer in the domain must have its own name, referred to as the host name. The combination of a host name, the enterprise domain name, and the Internet top-level domain name creates a name that is unique across the Internet. This unique name is referred to as the fully qualified domain name (FQDN). Each part of the FQDN is joined to the others by using the . character (the period, also referred to as a dot), creating a name that looks like this: www.microsoft.com.