The set of databases that performs the correspondence between an IP address and the fully qualified domain name. Note: The domain name system contains a kind of naming tree for IP addresses, hosts, and domains, identifying each Internet node unambiguously. 2. The online distributed database system that (a) is used to map human-readable addresses into Internet Protocol (IP) addresses, (b) has servers throughout the Internet to implement hierarchical addressing that allows a site administrator to assign machine names and addresses, (c) supports separate mappings between mail destinations and IP addresses, and (d) uses domain names that (i) consist of a sequence of names, i.e., labels, separated by periods, i.e., dots, (ii) usually are used to name Internet host computers uniquely, (iii) are hierarchical, and (iv) are processed from right to left, such as the host nic. Ddn. Mil has a name (nic -- the Network Information Center,) a subdomain (ddn -- the Defense Data Network,) and a primary domain (mil -- the MILNET. )