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 English 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).
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:
In addition, a group of internationalized domain name (IDN) top-level domains has been installed under test for testing purposes. Since November 2009, countries and territories may apply for IDN ccTLDs.
In October 2009, ICANN resolved to start accepting applications for top-level internationalized domain names from representatives of countries and territories starting in November 2009. Such IDN ccTLDs are domain names that are encoded in whole or in part as Unicode and represented in applications as language-specific scripts or alphabets.
Egypt, the Russian Federation, Saudi Arabia, and the United Arab Emirates were among the first countries to apply for the new internationalized domain name country code top-level domains and in January 2010 ICANN announced that these countries' IDN ccTLDs were the first four new IDN ccTLDs to have passed the Fast Track String Evaluation within the domain application process.
The domain arpa was the first Internet top-level domain. It was intended to be used only temporarily, aiding in the transition of traditional ARPANET host names to the domain name system. However, after it had been used for reverse DNS lookup, it was found impractical to retire it, and is used today exclusively for Internet infrastructure purposes such as in-addr.arpa for IPv4 and ip6.arpa for IPv6 reverse DNS resolution, uri.arpa and urn.arpa for the Dynamic Delegation Discovery System, and e164.arpa for telephone number mapping based on NAPTR DNS records. For historical reasons, arpa is sometimes considered to be a generic top-level domain.
RFC 2606 reserves the following four top-level domain names to avoid confusion and conflict. They may be used for various specific purposes however, with the intention that these should not occur in production networks within the global domain name system:
In the late 1980s InterNIC created the nato domain for use by NATO. NATO considered none of the then existing TLDs as adequately reflecting their status as an international organization. Soon after this addition, however, InterNIC also created the int TLD for the use by international organizations in general, and persuaded NATO to use the second level domain nato.int instead. The nato TLD, no longer used, was finally removed in July 1996.
Other historical TLDs are cs for Czechoslovakia (now cz for Czech Republic and sk for Slovak Republic), dd for East Germany (using de after reunification of Germany), yu for Federal Republic of Yugoslavia (now rs for Serbia and me for Montenegro), and zr for Zaire (now cd for Democratic Republic of the Congo). In contrast to these, the TLD su has remained active despite the demise of the Soviet Union that it represents.
About the time that ICANN discussed and finally introduced aero, biz, coop, info, museum, name, and pro TLDs, site owners argued that a similar TLD should be made available for adult and pornographic websites to settle the dispute of obscene content on the Internet and the responsibility of service providers under the Communications Decency Act of 1996. Several options were proposed including xxx, sex and adult, but ICANN has not created any.
During the 32nd International Public ICANN Meeting in Paris in 2008, ICANN started a new process of TLD naming policy to take a "significant step forward on the introduction of new generic top-level domains." This program envisions the availability of many new or already proposed domains, as well a new application and implementation process. Observers believed that the new rules could result in hundreds of new gTLDs to be registered. Proposed TLDs include music, shop, berlin and nyc.
ICANN's slow progress in creating new generic top-level domains, and the high application costs associated with TLDs, contributed to the creation of alternate DNS roots with different sets of top-level domains. Such domains may be accessed by configuration of a computer with alternate or additional (forwarder) DNS servers or plugin modules for web browsers. Browser plugins detect alternate root domain requests and access an alternate domain name server for such requests.
Several networks, such as BITNET, CSNET, UUCP or other networks, existed that were in widespread use among computer professionals and academic users, that were incompatible with the Internet and exchanged e-mail with the Internet via special e-mail gateways. For relaying purposes on the gateways, messages associated with these networks were labeled with suffixes such as bitnet, oz, csnet, or uucp, but these domains did not exist as top-level domains in the public Domain Name System of the Internet.
Most of these networks have long since ceased to exist, and although UUCP still gets significant use in parts of the world where Internet infrastructure has not yet become well-established, it subsequently transitioned to using Internet domain names, so pseudo-domains now largely survive as historical relics. One notable exception is the 2007 emergence of SWIFTNet Mail, which uses the swift pseudo-domain.
The top-level pseudo domain local is required by the Zeroconf protocol. It is also used by many organizations internally, which may become a problem for those users as Zeroconf becomes more popular. Both site and internal have been suggested for private usage, but no consensus has emerged.
The anonymity network Tor has a top-level pseudo-domain onion, which can only be reached with a Tor client because it uses the Tor-protocol (onion routing) to reach the hidden service to protect the anonymity of users.