A domain name is an identification label that defines a realm of
administrative autonomy, authority, or control on the Internet,
based on the Domain Name System (DNS).
Domain names are used in various networking contexts and
application-specific naming and addressing purposes. They are
organized in subordinate levels (sub domains) 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, NET 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
that wish to connect local area networks to the Internet, run web
sites, or create other publicly accessible Internet resources. The
registration of these domain names is usually administered by domain
name registrars who sell their services to the public.
Individual Internet host computers use domain names as host
identifiers, or hostnames. Hostnames are the leaf labels in the
domain name system usually without further subordinate domain name
space. Hostnames appear as a component in Uniform Resource Locators
(URLs) 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 (SIP), the
DomainKeys used to verify DNS domains in e-mail systems, and in many
other Uniform Resource Identifiers (URIs).
An important purpose of domain names is to provide easily
recognizable and memorial names to numerically addressed Internet
resources. This abstraction allows any resource (e.g., website) 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 often referred to simply 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.