A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a fully qualified domain name. For example, in the domain name www.example.com, the top-level domain is .com, or .COM, as domain names are not case-sensitive. Management of most top-level domains is delegated to responsible organizations by the Internet Corporation for Assigned Names and Numbers (ICANN), which operates the Internet Assigned Numbers Authority (IANA) and is in charge of maintaining the DNS root zone.
Originally, the top-level domain space was organized into three main groups, Countries, Categories, and Multiorganizations. An additional temporary group consisted only of the initial DNS domain, arpa, intended for transitional purposes toward the stabilization of the domain name system.
Countries are designated in the Domain Name System by their two-letter ISO country code; there are exceptions, however (e.g., .uk). This group of domains is therefore commonly known as country-code top-level domains (ccTLD). Since 2009, countries with non-Latin based alphabets or scripting systems may apply for internationalized country code top-level domain names, which are displayed in end-user applications in their language-native script or alphabet, but use a Punycode-translated ASCII domain name in the Domain Name System.
The Categories group has become known as the generic top-level domains. Initially this group consisted of GOV, EDU, COM, MIL, ORG, and NET.
In the growth of the Internet, it became desirable to create additional generic top-level domains. Some of the initial domains' purposes were also generalized, modified, or assigned for maintenance to special organizations affiliated with the intended purpose.
As a result, IANA today distinguishes the following groups of top-level domains:
country-code top-level domains (ccTLD): Two letter domains established for countries or territories. With some historical exceptions, the code for any territory is the same as its two-letter ISO 3166 code.
internationalized country code top-level domains (IDN ccTLD).
generic top-level domains (gTLD): Top-level domains with three or more characters
unsponsored top-level domains: domains that operate directly under policies established by ICANN processes for the global Internet community.
sponsored top-level domains (sTLD): These domains are proposed and sponsored by private agencies or organizations that establish and enforce rules restricting the eligibility to use the TLD. Use is based on community theme concepts.
infrastructure top-level domain: This group consists of one domain, the Address and Routing Parameter Area (ARPA). It is managed by IANA on behalf of the Internet Engineering Task Force for various purposes specified in the Request for Comments publications.
In addition, a group of internationalized domain name (IDN) top-level domains has been installed under test for testing purposes in the IDN development process.
The authoritative list of currently existing TLDs in the root zone is published at the IANA website at http://www.iana.org/domains/root/db/.
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