A domain name is an identification string that defines a realm of administrative autonomy, authority or control within the Internet. Domain names are formed by the rules and procedures of the Domain Name System, any name registered in the DNS is a domain name. Domain names are used in various networking contexts and application-specific naming and addressing purposes, in 2015,294 million domain names had been registered. Domain names are organized in levels of the DNS root domain. The registration of domain names is usually administered by domain name registrars who sell their services to the public. A fully qualified domain name is a name that is completely specified with all labels in the hierarchy of the DNS. A domain name may represent entire collections of 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 for Internet resources such as web sites, 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, the Domain Keys used to verify DNS domains in e-mail systems, 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 topology of the network. Such a move usually requires changing the IP address of a resource, Domain names are used to establish a unique identity. Organizations can choose a name that corresponds to their name. A generic domain is a name that defines a category, rather than a specific or personal instance, for example. Some examples of names are books. com, music. com. Companies have created brands based on names, and such generic domain names may be valuable. The use of names in commerce may subject them to trademark law. The practice of using a simple memorable abstraction of a hosts numerical address on a computer network dates back to the ARPANET era, before the advent of todays commercial Internet
Image: Website seizure notice
Image: Liberty Reserve seizure
The hierarchical domain name system, organized into zones, each served by domain name servers.