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The top-level domain (TLD) refers to the highest level in the Domain Name System of the internet. It is the last part of the domain name. For example, in www.example.com, .com is the TLD. Domain names are not case sensitive so .COM works the same as .com. The management of most TLDs are assigned by the Internet Corporation for Assigned Names and Numbers (ICANN), which operates the Internet Assigned Numbers Authority (IANA) and maintains the DNS root zone.
The original top level domain space was organized into three groups, Countries, Categories, and Multiorganizations, with an additional temporary group: Address and Routing Parameter Area (ARPA). This group was intended for transitional purposes toward the stabilization of the domain name system.
The IANA distinguishes the following groups of top level domains:
Country code (ccTLD) these two letters are established for countries or territories using their two-letter ISO country code.
Internationalized country code (IDNccTLD) used in non-Latin character sets, like Arabic or Chinese.
Generic (gTLDs) are top level domains with three or more characters, examples are .gov, .edu, .com, .mil, .org, .net, and recently added .info.
Sponsored (TLDs) are domains sponsored by private agencies or organizations that establish and enforce rules for the eligibility of the TLD based on community concepts.
Infrastructure is the group consisting of one domain ARPA. This domain was the first TLD, and it was intended to be only used temporarily. However, it was for reverse DNS lookup so it was kept active, and today it is used exclusively for internet infrastructure purposes.
Example domain; invalid domain; localhost domain; and test domain.
NATO in the late 1980’s wanted a TLD unique to their status, so .nato was created as a TLD specifically for the organization, but when the TLD .int was created, NATO was persuaded to use nato.int instead. The .nato TLD was removed and no longer used after 1996.
Late 2000 several new TLDs were introduced, .aero, .biz, .coop, .info, .museum, .name, and .pro. Several other proposed TLDs were .arts, .firm, .nom, .rec, .shop, and .web. In June 2010
.xxx was approved by the ICANN for use for sites with adult content. As of June 13, 2012, ICANN has revealed 2,000 applications for new TLDs and are expected to be thoroughly examined and if approved in be used starting in 2013.
The high cost of a TLD application along with ICANN’s slowness of producing new gTLDs led to the creation of alternate DNS roots with a different set of TLDs. These are accessible by configuring a computer with alternate or additional DNS servers, or plugin modules for the web browser. The browser will detect alternate root domain requests and then access an alternate domain name server to handle the request.
A few networks used .bitnet, .oz, .csnet, and .uucp, but were not part of the public TLD system. These are called pseudo-domains, used by professionals and academic users to exchange e-mail with special gateways. Most of these networks are no longer in use, except .uucp which still is used in parts of the world where the internet infrastructure is not well established. In 2007 SWIFTNet Mail emerged and uses the swift pseudo-domain.