the hierarchy of labels in a domain name.|thumb  a domain name is a unique name that identifies an internet resource such as a website. it is an identification string that defines a realm of administrative autonomy, authority or control on the internet. domain names are formed by the rules of the domain name system (dns). any name registered in the dns is a domain name. the functional description of domain names is presented in the domain name system article. broader usage and industry aspects are captured here. domain names are used in various networking contexts and application-specific naming and addressing purposes. in general, a domain name represents an internet protocol (ip) resource, such as a personal computer used to access the internet, a server computer hosting a web site, or the web site itself or any other service communicated via the internet. in 2014, the number of active domains reached 271 million. 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 in the hierarchy of the dns, having no parts omitted. domain names are usually written in lowercase, although labels in the domain name system are case-insensitive. ==purpose== domain names serve as more easily memorable names for internet resources such as computers, networks, and services. a domain name represents an internet protocol (ip) resource. individual internet host computers use domain names as host identifiers, or host names. host names are 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 pr
Domain_name_system Identifiers