Domain name

A domain name is a way that hosts, servers, websites, and other things on the Internet are generally identified. They translate to IP addresses, the base addressing system of the Internet, via a protocol called DNS, but are designed to be more human-meaningful and stable names than the all-numeric IP addresses.

Domain names replaced the earlier ARPAnet hostname system, where each network host had a separate centrally-assigned address. Instead, DNS uses a distributed system where domain names can be registered under one of a number of top level domains which can operate independent registries, and in turn each domain can assign an unlimited number of subdomains within it. Names have the dot (.) as a separator between levels and are read from the rightmost part leftward, so that host.subdomain.example.net has net as its top-level domain, example as its second-level domain (registered under .net), subdomain as a third-level subdomain (assigned by whoever controls example.net with no further registry/registrar action necessary), and host as the hostname beneath that subdomain.

Originally, generic domain names were grouped by category of entity (commercial, organization, government, etc.) into a small group of three-letter top level domains, while two-letter top-level domains were reserved for country codes. One anomalous four-letter domain, .arpa, was originally used as a temporary place for ARPAnet hosts not yet assigned a permanent domain, but survived as the location of in-addr.arpa, an address used for reverse domain lookups from IP addresses.

However, after the year 2000, a number of new top-level domains were added, some of which had more letters in them (e.g., .museum), and as of 2013 a huge expansion is in progress which is expected to see new domain endings by the hundreds.

Nevertheless, simple-minded people tend to expect that every address ends in .com, even though this technically was only intended to apply to commercial entities.

Official documents

 * RFC 289 (hostnames before DNS)
 * RFC 819 (early description of DNS before domains even began to be assigned)
 * RFC 882 (formal introduction of domain names)
 * RFC 1034 (later description)
 * RFC 1591 (official descriptions of the original group of top level domains)
 * RFC 2606 (dummy domains reserved for test/example use)
 * RFC 3071 (reflections on domain name system)
 * UDRP (policy for domain disputes)

Official sites

 * ICANN

Other links

 * Dan's Domain Name Site