Each subdomain can have further levels of hierarchy as necessary to specify the specific services offered by that organization via the Internet network. Domain names are only half the story when you are talking about internet addresses. Names are convenient for people, but are not as useful to computer systems. Computer systems are just big calculators at heart and work much more efficiently with numbers than with names. So every Internet address name is associated with a numeric Internet address.
Numeric addresses are four sets of three- digit numbers separated by periods.
Most people remember names much more easily than numbers, and names provide an associative reference that helps the memory process and places an object within a learned framework of concepts. That's why at the heart of any Internet network is a system to translate the names that are useful to people into the numbers that are useful to computers. DNS consists of a database of names and numbers as well as software to communicate those names and numbers between different computers. Each domain, like unt.edu, has one central computer repository for its names and addresses, although this information can be duplicated on other computers to provide a backup service or increase network efficiency. These repositories of Internet addresses are called DNS servers.