A top-level domain (TLD) is one of the domains
at the highest level in the hierarchical Domain Name System
of the Internet
after the root domain.
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 non empty label of a fully qualified domain name
. For example, in the domain name www.example.com.
, the top-level domain is com
. Responsibility for management of most top-level domains is delegated to specific organizations by the Internet Governance Forum
(IGF) an Internet multi-stakeholder community, 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 of only the initial DNS domain, arpa
, and was intended for transitional purposes toward the stabilization of the domain name system.
IANA distinguishes the following groups of top-level domains:
* Infrastructure top-level domain
(ARPA): This group consists of one domain, the Address and Routing Parameter Area
. It is managed by IANA on behalf of the Internet Engineering Task Force
for various purposes specified in the Request for Comments
* Generic top-level domain
s (gTLD): Top-level domains with three or more characters
* restricted generic top-level domains (grTLD): These domains are managed under official ICANN accredited registrars.
* Sponsored top-level domain
s (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; these domains are managed under official ICANN accredited registrars.
* country-code top-level domain
s (ccTLD): Two-letter domains established for countries
. With some historical exceptions, the code for any territory is the same as its two-letter ISO 3166
** Internationalized country code top-level domain
s (IDN ccTLD): ccTLDs in non-Latin character sets (e.g., Arabic, Cyrillic, Hebrew, or Chinese).
* Test top-level domains
(tTLD): These domains were installed under .test
for testing purposes in the IDN development process; these domains are not present in the root zone.
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 domain
s (ccTLD). Since 2009, countries with non–Latin-based scripts 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.
Generic top-level domains (formerly ''categories'') initially consisted of gov
, and net
. More generic TLDs have been added, such as info
The authoritative list of current TLDs
in the root zone is published at the IANA website ahttps://www.iana.org/domains/root/db/
Internationalized country code TLDs
An internationalized country code top-level domain
(IDN ccTLD) is a top-level domain with a specially encoded domain name that is displayed in an end user application, such as a web browser
, in its language-native script or alphabet
(such as the Arabic alphabet
), or a non-alphabetic writing system
(such as Chinese character
s). IDN ccTLDs are an application of the internationalized domain name
(IDN) system to top-level Internet domains assigned to countries, or independent geographic regions.
ICANN started to accept applications for IDN ccTLDs in November 2009, and installed the first set into the Domain Names System in May 2010. The first set was a group of Arabic names for the countries of Egypt, Saudi Arabia, and the United Arab Emirates. By May 2010, 21 countries had submitted applications to ICANN, representing 11 scripts.
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.
A set of domain names is reserved by the Internet Engineering Task Force as special-use domain name
s per authority of Request for Comments (RFC) 6761. The practice originated in RFC 1597 for reserved address allocations in 1994, and reserved top-level domains in RFC 2606 of 1999. RFC 6761 reserves the following four top-level domain names to avoid confusion and conflict.
[RFC 6761, ''Special-Use Domain Names'', S. Cheshire, M. Krochmal, The Internet Society (February 2013)]
Any such reserved usage of those TLDs should not occur in production networks that utilize the global domain name system:
: reserved for use in examples
: reserved for use in invalid domain names
: reserved to avoid conflict with the traditional use of localhost
as a hostname
: reserved for use in tests
RFC 6762 reserves the use of .local
for link-local host names that can be resolved via the Multicast DNS
name resolution protocol.
RFC 7686 reserves the use of .onion
for the self-authenticating names of Tor hidden services
. These names can only be resolved by a Tor client because of the use of onion routing
to protect the anonymity of users.
proposes reserving the use of .internal for "names which do not have meaning in the global context but do have meaning in a context internal to their network", and for which the RFC 6761 reserved names are semantically inappropriate.
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
(now using cz
for Czech Republic
for East Germany
after reunification of Germany
for SFR Yugoslavia
and Serbia and Montenegro
(now using ba
for Bosnia and Herzegovina
for North Macedonia
), and zr
for the 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. Under the chairmanship of Nigel Roberts
is working on a policy for retirement of ccTLD
s that have been removed from ISO 3166
Around late 2000, ICANN
discussed and finally introduced aero
, 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, to address the responsibility of US service providers under the US Communications Decency Act
of 1996. Several options were proposed including xxx
, ''sex'' and ''adult''. The .xxx top-level domain eventually went live in 2011.
An older proposal consisted of seven new gTLDs: arts, firm, info
, nom, rec, shop
, and web
. Later biz
, and name
covered most of these old proposals.
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 envisioned the availability of many new or already proposed domains, as well as a new application and implementation process. Observers believed that the new rules could result in hundreds of new gTLDs being registered.
On 13 June 2012, ICANN
announced nearly 2,000 applications for top-level domains, which began installation throughout 2013. Donuts Inc.
invested $57 million in more than 300 applications
while Famous Four Media
applied for 61 new domains. The first seven – ''bike'', ''clothing'', ''guru'', ''holdings'', ''plumbing'', ''singles'', and ''ventures'' – were released in 2014.
Several networks, such as BITNET
, and UUCP
, existed that were in widespread use among computer professionals and academic users, but were not interoperable directly with the Internet and exchanged mail with the Internet via special email gateways. For relaying purposes on the gateways, messages associated with these networks were labeled with suffixes such as bitnet
, 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, and 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 anonymity network Tor
formerly used the top-level pseudo-domain onion
for Tor hidden service
s, which can only be reached with a Tor client because it uses the Tor onion routing
protocol to reach the hidden service to protect the anonymity of users. However, the pseudo-domain became officially reserved in October 2015. i2p
provides a similar hidden pseudo-domain, .i2p.
hubs use the top-level pseudo-domain home for local DNS resolution of routers, modems and gateways.
* List of Internet top-level domains
* Alternative DNS root
* Domain hack
* Domain name registrar
* Public Suffix List
* Second-level domain
''Addressing the World''
''National Identity and Internet Country Code Domains'', edited by Erica Schlesinger Wass (Rowman & Littlefield, 2003, ) examines connections between cultures and their ccTLDs.
''Ruling the Root''
by Milton Mueller (MIT Press, 2001, ) discusses TLDs and domain name policy more generally.
IANA TLD List IANA List of TLDs on the DNS Root ZoneArticles on CircleID about TLDs
TLDs accepted in 2012
Category:Domain Name System