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.
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Use of domain names in file formats
Domain names appear in many roles in file formats. Often the value of some data element will be a URL (or URI or URN or IRI, to name some other related things which have technical distinctions not always adhered to in usage), and most forms of URL have a domain name in them (e.g., the common HTTP variety starts with http:// followed by a hostname usually expressed in domain form). E-mail addresses also contain domain names following the @ sign.
A common system for generating unique identifiers (e.g., names of custom queues in iOS apps programmed under Cocoa in Objective-C; but similar schemes turn up in many places where identifiers are needed) is to begin the name with a reversed domain name based on the company or organization using the identifier, so that, for instance, if your company owns the domain example.com, your identifiers would start with com.example. This ensures that they don't clash with identifiers used by any other company; you merely need to adopt whatever internal policies are needed to prevent name clashes within your company. The reversed order is used to put highest-level parts first, since the low-level identifier you are creating comes at the end of the string. (Interestingly, the British-based academic network JANET used such a reversed naming system back in the 1980s, with hostnames starting with uk., but they had to flip the order the other way when they joined the Internet and had to follow its naming conventions.)
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)
