The top-level domains in the Internet's DNS are comprised of two-letter abbreviations for each country, such as "UK" and "US", as well as a handful of special domains that exist mainly due to tradition, such as "EDU", "COM", "NET", "GOV", and "MIL".
Another important task of DNS on the Internet is reverse name resolution, or the translation of IP addresses back to domain names. This is important, for example, so servers can determine and record the full domain name of machines connecting to them over the network. It is not efficient to use the same set of DNS records for reverse name resolution (one would have to search through *every* domain name on the Internet in hopes of finding the one with the matching IP address!). Instead, a seperate domain called "IN-ADDR.ARPA" has been set aside to provide a hierarchy for translating IP addresses into names. A DNS lookup of "borg.ee.msstate.edu" would reveal it has the IP address "130.18.64.5". If one has the IP address and wishes to know the name, one must perform a DNS lookup of "5.64.18.130.in-addr.arpa", which will return the name. Reverse name resolution fields use the PTR resource record, which "points" to the correct position in the normal DNS space. The hierarchy under "IN-ADDR.ARPA" can be delegated, of course, just like any other domain. This graphic demonstrates the principle of reverse name resolution.
There are a number of "root nameservers" in existance in various corners of the Internet which store the ultimate information for the root domain, as well as zones for a handful of top-level domains. Certain organizational units such as countries and universities have delegation of domains underneath the root and top-level domains. Entities wishing domain names must register, and perhaps receive delegation of, their domains from the appropriate registry.
