Domain names are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. The first-level set of domain names are the top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains com, info, net, edu, and org, and the country code top-level domains (ccTLDs). Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users who wish to connect local area networks to the Internet, create other publicly accessible Internet resources or run web sites. The registration of these domain names is usually administered by domain name registrars who sell their services to the public. A fully qualified domain name (FQDN) is a domain name that is completely specified with all labels in the hierarchy of the DNS, having no parts omitted. Labels in the Domain Name System are case-insensitive, and may therefore be written in any desired capitalization method, but most commonly domain names are written in lowercase in technical contexts.
Domain names serve to identify Internet resources, such as computers, networks, and services, with a text-based label that is easier to memorize than the numerical addresses used in the Internet protocols. A domain name may represent entire collections of such resources or individual instances. Individual Internet host computers use domain names as host identifiers, also called host names. The term host name is also used for the leaf labels in the domain name system, usually without further subordinate domain name space. Host names appear as a component in Uniform Resource Locators (URLs) for Internet resources such as web sites (e.g., en.wikipedia.org).
Domain names are also used as simple identification labels to indicate ownership or control of a resource. Such examples are the realm identifiers used in the Session Initiation Protocol (SIP), the Domain Keys used to verify DNS domains in e-mail systems, and in many other Uniform Resource Identifiers (URIs). An important function of domain names is to provide easily recognizable and memorizable names to numerically addressed Internet resources. This abstraction allows any resource to be moved to a different physical location in the address topology of the network, globally or locally in an intranet. Such a move usually requires changing the IP address of a resource and the corresponding translation of this IP address to and from its domain name. Domain names are used to establish a unique identity. Organizations can choose a domain name that corresponds to their name, helping Internet users to reach them easily. A generic domain is a name that defines a general category, rather than a specific or personal instance, for example, the name of an industry, rather than a company name. Some examples of generic names are books.com, music.com, and travel.info. Companies have created brands based on generic names, and such generic domain names may be valuable. Domain names are often simply referred to as domains and domain name registrants are frequently referred to as domain owners, although domain name registration with a registrar does not confer any legal ownership of the domain name, only an exclusive right of use for a particular duration of time. The use of domain names in commerce may subject them to trademark law.
1982 January As described in Computer Mail Meeting Notes, RFC 805, it was initially the need for a real-world solution to the complexity of email relaying that triggered the development of the domain concept. A group of ARPANET researchers, principles, and related parties held a meeting in January, 1982, to discuss a solution foremail relaying. As described on the email addresses page, email was often originally sent from site to site toits destination along a path of systems, and might need to go through a half a dozen or more links that would connect at certain times of the day.
To send an email to someone, you had to first be a human router and specify a valid path to the destination aspart of the address. If you didn’t know a valid route, the software couldn’t help you. In order to solve this problem, domain names were created to provide each person with one address regardless of where email was sent from. As RFC 805 put it, “The hierarchical domain type naming differs from source routing in that the former gives absolute addressing while the latter gives relative addressing”. RFC 805 outlines many of the basic principles of the eventual domain name system, including the need for top level domains to provide a starting point for delegation of queries, the need for second level domains to be unique — and therefore the requirement for a registrar type of administration, and the recognition that distribution of individual name servers responsible for each domain would provide administration and maintenance advantages. Within the year, the concept was developed through a series of communications. In March, the hosts table definition was updated with DoD Internet Host Table Specification, RFC 810, and NIC’s introduction of a server function to provide individual hostname / address translations was described in Hostnames Server, RFC 811, both documents including the domain concept. In August, The Domain Naming Convention for Internet User Applications, RFC 819, provided an excellent overview of the concept. And then, in October, the full concept of a distributed system of name servers, each serving its local domain,was described in A Distributed System for Internet Name Service, RFC 830, providing the main architectural outlines of the system still in use today.
1982 February 8th The conclusion in this area wasthat the current “user@host” mailbox identifier should be extended to”email@example.com” where “domain” could be a hierarchy ofdomains.- J. Postel; Computer Mail Meeting Notes, RFC 805; 8 Feb 1982. TheDomain Name System was originally invented to support the growth of email communications on the ARPANET, and now supports the Internet on a global scale. Alphabetic host names were introduced on the ARPANET shortly after its creation, and greatly increased usability since alphabetic names are much easier to remember than semantically meaningless numeric addresses. Host names were also useful for developmentof network-aware computer programs, since they could reference a constant host name without concern about changes to the physical address due to network alterations. Of course, the infrastructure of the underlying network was still based on numeric addresses, so each site maintained a “HOSTS.TXT” file that provided a mapping between host names and network addresses in a set of simple text records that could be easily read by a person or program.