Top-level domain

"TLD" and "TLDN" redirect here. For Temporary Location Directory Number, seeMobile Station Roaming Number.For other uses, seeTLD (disambiguation).

See also:Country code top-level domain

Examples of theover 1,500 TLDs

Example domain	Type	Sponsoring institution
.arpa	Infrastructure	Internet Architecture Board;restricted
.blue	Generic	Identity Digital Limited;unrestricted
.ovh	Generic	OVH SAS;run byAFNIC,unrestricted
.name	Restricted generic	VeriSign Information Services, Inc.;unrestricted
.ac	Country-code	Internet Computer Bureau;unrestricted
.zw	Country-code	Postal and Telecommunications Regulatory Authority of Zimbabwe;run byTelOne Zimbabwe;unrestricted
.aero	Sponsored	Société Internationale de Télécommunications Aéronautiques;unrestricted
.ไทย	Internationalized country-code	THNIC

Atop-level domain(TLD) is one of thedomainsat the highest level in the hierarchicalDomain Name Systemof theInternetafter the root domain.The top-level domain names are installed in theroot zoneof the name space. For all domains in lower levels, it is the last part of thedomain name,that is, the last non-empty label of afully qualified domain name.For example, in the domain nameexample,the top-level domain is.Responsibility for management of most top-level domains is delegated to specific organizations by theICANN,an Internet multi-stakeholder community, which operates theInternet Assigned Numbers Authority(IANA), and is in charge of maintaining theDNS root zone.

History

Originally, the top-level domain space was organized into three main groups:Countries,Categories,andMultiorganizations.An additionaltemporarygroup consisted of only the initial DNS domain,.arpa,and was intended for transitional purposes toward the stabilization of the domain name system.

Types

As of 2015,IANA distinguishes the following groups of top-level domains:

• Infrastructure top-level domain(ARPA): This group consists of one domain, theAddress and Routing Parameter Area.It is managed by IANA on behalf of theInternet Engineering Task Forcefor various purposes specified in theRequest for Commentspublications.
• Generic top-level domains(gTLD): Top-level domains with three or more characters
• Generic restricted top-level domains (grTLD): These domains are managed under official ICANN-accredited registrars.
• 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; these domains are managed under official ICANN accredited registrars.
• country-code top-level domains(ccTLD): Two-letter domains established forcountriesorterritories.With some historical exceptions, the code for any territory is the same as its two-letterISO 3166code.
  • Internationalized country code top-level domains(IDN ccTLD): ccTLDs in non-Latin character sets (e.g., Arabic, Cyrillic, Greek, Hebrew, or Chinese).
• Test top-level domains(tTLD): These domains were installed under.testfor 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 theirtwo-letter ISO country code;there are exceptions, however (e.g.,.uk). This group of domains is, therefore, commonly known ascountry-code top-level domains(ccTLD). Since 2009, countries with non–Latin-based scripts may apply forinternationalized country code top-level domainnames, which are displayed in end-user applications in their language-native script or Alpha bet, but use aPunycode-translated ASCII domain name in the Domain Name System.

Generic top-level domains (formerlycategories) initially consisted of.gov,.edu,,.mil,.org,and.net.More generic TLDs have been added, such as.info.

The authoritativelist of current TLDsin the root zone is published at the IANA website athttps:// iana.org/domains/root/db/.

Internationalized country code TLDs

Aninternationalized 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 aweb browser,in its language-native script orAlpha bet(such as theArabic Alpha bet), or a non- Alpha beticwriting system(such asChinese characters). IDN ccTLDs are an application of theinternationalized 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.

Infrastructure domain

The domain.arpawas 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 forreverse DNS lookup,it was found impractical to retire it, and is used today exclusively for Internet infrastructure purposes such as in-addr.arpa forIPv4and ip6.arpa for IPv6 reverse DNS resolution, uri.arpa and urn.arpa for theDynamic Delegation Discovery System,and e164.arpa fortelephone number mappingbased onNAPTR DNS records.For historical reasons,.arpais sometimes considered to be a generic top-level domain.

Reserved domains

A set of domain names is reservedby the Internet Engineering Task Force asspecial-use domain namesper 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.Any such reserved usage of those TLDs should not occur in production networks that utilize the global domain name system:

• .example– reserved for use in examples
• .invalid– reserved for use in invalid domain names
• .localhost– reserved to avoid conflict with the traditional use oflocalhostas a hostname
• .test– reserved for use in tests

RFC 6762 reserves the use of.localfor link-local host names that can be resolved via themulticast DNSname resolution protocol.

RFC 7686 reserves the use of.onionfor the self-authenticating names ofToronion services.These names can only be resolved by a Tor client because of the use ofonion routingto protect the anonymity of users.

ICANN proposed the use of.internalon 2024-01-24with request for feedback. In 2017 an Internet-Draftdraft-wkumari-dnsop-internal-00already proposed reserving the use of.internalfor "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.

Historical domains

In the late 1980s,InterNICcreated the.natodomain for use byNATO.NATO considered none of the then-existing TLDs as adequately reflecting their status as aninternational organization.Soon after this addition, however, InterNIC also created the.intTLD for the use by international organizations in general, and persuaded NATO to use the second level domainnato.intinstead. ThenatoTLD, no longer used, was finally removed in July 1996.

Other historical TLDs are.csforCzechoslovakia(now using.czforCzech Republicand.skforSlovakia),.ddforEast Germany(using.deafterreunification of Germany),.yuforSFR YugoslaviaandSerbia and Montenegro(now using.baforBosnia and Herzegovina,.hrforCroatia,.meforMontenegro,.mkforNorth Macedonia,.rsforSerbiaand.siforSlovenia),.zrforZaire(now.cdfor theDemocratic Republic of the Congo), and.anforNetherlands Antilles(now.awforAruba,.cwforCuraçaoand.sxforSint Maarten). In contrast to these, the TLD.suhas remained active despite the demise of theSoviet Unionthat it represents. Under the chairmanship ofNigel Roberts,ICANN'sccNSOis working on a policy for the retirement ofccTLDsthat have been removed fromISO 3166.

Proposed domains

Further information:Proposed top-level domain

Around late 2000,ICANNdiscussed and finally introduced.aero,.biz,.coop,.info,.museum,.name,and.proTLDs. 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 USCommunications Decency Actof 1996. Several options were proposed includingxxx,sexandadult.The.xxxtop-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,.info,.museum,and.namecovered 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,ICANNannounced nearly 2,000 applications for top-level domains, which began installation throughout 2013.The first seven –bike,clothing,guru,holdings,plumbing,singles,andventures– were released in 2014.

Rejected domains

ICANN rejected several proposed domains to include.homeand.corpdue to conflicts regarding gTLDs that are in use in internal networks.

Investigation into the conflicts was conducted at ICANN's request byInterisle Consulting.The resulting report was to become known as the Name Collisionissue, which was first reported at ICANN 47.

Dotless domains

.org[.]is a node in the DNS tree, just likewikipedia.[org.]anden.[wikipedia.org.].As such, it has its ownDNS records.

Due to the structure ofDNS,each node in the tree has its own collection ofrecords,and since top-level domains are nodes in DNS, they have records of their own. For example, queryingorgitself (with a tool such asdig,host,ornslookup) returns information on itsnameservers:

QUESTION
org. IN ANY
ANSWER
org. 21599 IN NS a0.org.afilias-nst.info.
org. 21599 IN NS a2.org.afilias-nst.info.
org. 21599 IN NS b0.org.afilias-nst.org.
org. 21599 IN NS b2.org.afilias-nst.org.
[…]

Dotless domainsare top-level domains that take advantage of that fact, and implementA,AAAAorMXDNS records to serve webpages or allow incoming email directly on a TLD – for example, a webpage hosted onhttp://example/,or an email addressuser@example.

ICANNandIABhave spoken out against the practice, classifying it as a security risk among other concerns.ICANN's Security and Stability Advisory Committee (SSAC) additionally claims thatSMTP"requires at least two labels in theFQDNof a mail address "and, as such, mail servers would reject emails to addresses with dotless domains.

ICANN has also published a resolution in 2013 that prohibits the creation of dotless domains ongTLDs.ccTLDs,however, fall largely under their respective country'sjurisdiction,and not under ICANN's. Because of this, there have been many examples of dotless domains on ccTLDs in spite of ICANN's vocal opposition.

As of September 2023, that is the case of:

• Anguilla's.ai,online athttp://ai./(Archived22 January 2023 atarchive.today)
  It simply displays a notice that the website is no longer public.
• Uzbekistan's.uz,online athttps://uz./(Archived4 September 2023 atarchive.today)
  It's a mirror ofhttps://cctld.uz/,albeit with an invalid certificate.

Other ccTLDs with A or AAAA records, as of September 2023, include:.cm,.tkand.ws.

A similar query toorg's presented above can be made forai,which shows A and MX records for the TLD:

QUESTION
ai. IN ANY
ANSWER
ai. 21599 IN A 209.59.119.34
ai. 21599 IN MX 10 mail.offshore.ai.
ai. 21599 IN NS anycastdns1-cz.nic.ai.
ai. 21599 IN NS anycastdns2-cz.nic.ai.
ai. 21599 IN NS pch.whois.ai.
[…]

Historically, many other ccTLDs have had A or AAAA records. On 3 September 2013, as reported by theIETF,they were the following:.ac,.dk,.gg,.io,.je,.kh,.sh,.tm,.to,and.vi.

New TLDs

Following a 2014 resolution by ICANN, newly registered TLDs must implement the following A, MX, TXT, and SRV apex DNS records – where<TLD>stands for the registered TLD – for at least 90 days:

<TLD>. 3600 IN MX 10 your-dns-needs-immediate-attention.<TLD>.
<TLD>. 3600 IN SRV 10 10 0 your-dns-needs-immediate-attention.<TLD>.
<TLD>. 3600 IN TXT "Your DNS configuration needs immediate attention see https://icann.org/namecollision"
<TLD>. 3600 IN A 127.0.53.53

This requirement is meant to avoid domain name collisions when new TLDs are registered. For example, programmers may have used custom local domains such asfoo.barortest.dev,which would both collide with the creation of gTLDs.barin 2014 and.devin 2019. As of September 2023, the only top-level domains with these special apex records are.araband.music.

While this does create apex DNS records of type A and MX, they do not qualify as a dotless domain, as the records should not point to real servers. For instance, the A record contains the IP 127.0.53.53, a loopback address (seeIPv4 § Addressing), picked as a mnemonic to indicate a DNS-related problem, asDNS uses port 53.

Pseudo-domains

Main article:Pseudo-top-level domain

Several networks, such asBITNET,CSNET,andUUCP,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,.oz,.csnet,or.uucp,but these domains did not exist as top-level domains in the publicDomain Name Systemof 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 ofSWIFTNetMail, which uses the swift pseudo-domain.

The anonymity networkTorformerly used the top-level pseudo-domain.onionforonion services,which can only be reached with a Tor client because it uses the Toronion routingprotocol to reach the hidden service to protect the anonymity of users. However, the pseudo-domain became officially reserved in October 2015.i2pprovides a similar hidden pseudo-domain,.i2p. AndNamecoinuses the.bit pseudo-domain.

See also

• Alternative DNS root
• Domain hack
• Domain name registrar
• List of Internet top-level domains
• Public Suffix List
• Second-level domain

Further reading

• Addressing the World:National Identity and Internet Country Code Domains,edited by Erica Schlesinger Wass (Rowman & Littlefield, 2003,ISBN0-7425-2810-3) examines connections between cultures and their ccTLDs.
• Ruling the Rootby Milton Mueller (MIT Press, 2001,ISBN0-262-13412-8) discusses TLDs and domain name policy more generally.

External links

• Root Zone Database– list of TLDs on the DNS Root Zone, IANA
• IANA TLD List(plain text, ALL-CAPS)
• New TLDs– Articles on CircleID about TLDs
• "Top-Level Domain Names by Host Count".ISC.January 2008.Retrieved7 August2008.
• TLDs accepted in 2012(archived)