Домен верхнего уровня ( TLD ) — один из доменов на самом высоком уровне в иерархической системе доменных имен Интернета после корневого домена. [ 1 ] Доменные имена верхнего уровня устанавливаются в корневой зоне пространства имен. Для всех доменов на более низких уровнях это последняя часть доменного имени , то есть последняя непустая метка полностью квалифицированного доменного имени . Например, в доменном имени www.example.com доменом верхнего уровня является .com . Ответственность за управление большинством доменов верхнего уровня делегирована конкретным организациям ICANN , многосторонним интернет-сообществом, которое управляет Управлением по распределению номеров Интернета (IANA) и отвечает за поддержание корневой зоны DNS .
Первоначально пространство доменов верхнего уровня было организовано в три основные группы: Страны , Категории и Мультиорганизации . [2] Дополнительная временная группа состояла только из первоначального домена DNS, .arpa , [3] и была предназначена для переходных целей в направлении стабилизации системы доменных имен.
По состоянию на 2015 год [update]IANA выделяет следующие группы доменов верхнего уровня: [4]
Страны обозначаются в системе доменных имен двухбуквенным кодом страны ISO ; [5] однако есть исключения (например, .uk ). Поэтому эта группа доменов обычно известна как домены верхнего уровня с кодом страны (ccTLD). С 2009 года страны с нелатинскими алфавитами могут подавать заявки на интернационализированные доменные имена верхнего уровня с кодом страны , которые отображаются в приложениях конечных пользователей в их родном языке или алфавите, но используют доменное имя ASCII, переведенное в Punycode, в системе доменных имен.
Общие домены верхнего уровня (ранее категории ) изначально состояли из .gov , .edu , .com , .mil , .org и .net . Были добавлены более общие TLD, такие как .info .
Официальный список текущих доменов верхнего уровня в корневой зоне опубликован на сайте IANA по адресу https://www.iana.org/domains/root/db/.
Интернационализированный национальный домен верхнего уровня (IDN ccTLD) — это домен верхнего уровня со специально закодированным доменным именем, которое отображается в приложении конечного пользователя, например, в веб-браузере , на его родном языке, с использованием алфавита или письменности (например, арабского алфавита ) или неалфавитной системы письма (например, китайских иероглифов ). IDN ccTLD — это приложение системы интернационализированных доменных имен (IDN) к интернет-доменам верхнего уровня, назначенным странам или независимым географическим регионам.
ICANN начала принимать заявки на IDN ccTLD в ноябре 2009 года, [6] и установила первый набор в систему доменных имен в мае 2010 года. Первый набор представлял собой группу арабских имен для стран Египта, Саудовской Аравии и Объединенных Арабских Эмиратов. К маю 2010 года 21 страна подала заявки в ICANN, представляя 11 алфавитов. [7]
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.[8]
A set of domain names is reserved[9][10] by the Internet Engineering Task Force as special-use domain names 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.[11] Any such reserved usage of those TLDs should not occur in production networks that utilize the global domain name system:
RFC 6762 reserves the use of .local for link-local host names that can be resolved via the multicast DNS name resolution protocol.[12]
RFC 7686 reserves the use of .onion for the self-authenticating names of Tor onion services. These names can only be resolved by a Tor client because of the use of onion routing to protect the anonymity of users.[13]
ICANN reserved .internal for private-use applications on July 29, 2024.[14]
In the late 1980s, InterNIC created the .nato domain for use by NATO.[citation needed] 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.[citation needed]
Other historical TLDs are .cs for Czechoslovakia (now using .cz for Czech Republic and .sk for Slovakia), .dd for East Germany (using .de after reunification of Germany), .yu for SFR Yugoslavia and Serbia and Montenegro (now using .ba for Bosnia and Herzegovina, .hr for Croatia, .me for Montenegro, .mk for North Macedonia, .rs for Serbia and .si for Slovenia), .zr for Zaire (now .cd for the Democratic Republic of the Congo), and .an for Netherlands Antilles (now .aw for Aruba, .cw for Curaçao and .sx for Sint Maarten). 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, ICANN's ccNSO is working on a policy for the retirement of ccTLDs that have been removed from ISO 3166.
Around late 2000, ICANN discussed and finally introduced[15] .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, 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.[16] The .xxx top-level domain eventually went live in 2011.[17]
An older proposal consisted of seven new gTLDs: arts, firm, .info, nom, rec, .shop, and .web.[18] Later .biz, .info, .museum, 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".[19] This program envisioned the availability of many new or already proposed domains, as well as a new application and implementation process.[20] Observers believed that the new rules could result in hundreds of new gTLDs being registered.[21]
On 13 June 2012, ICANN announced nearly 2,000 applications for top-level domains, which began installation throughout 2013.[22][23] The first seven – bike, clothing, guru, holdings, plumbing, singles, and ventures – were released in 2014.[24]
ICANN rejected several proposed domains to include .home and .corp due to conflicts regarding gTLDs that are in use in internal networks.
Investigation into the conflicts was conducted at ICANN's request by Interisle Consulting. The resulting report was to become known as the Name Collision[25] issue, which was first reported at ICANN 47.[26]
Due to the structure of DNS, each node in the tree has its own collection of records, and since top-level domains are nodes in DNS, they have records of their own. For example, querying org itself (with a tool such as dig, host, or nslookup) returns information on its nameservers:
QUESTIONorg. IN ANYANSWERorg. 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 domains are top-level domains that take advantage of that fact, and implement A, AAAA or MX DNS records to serve webpages or allow incoming email directly on a TLD – for example, a webpage hosted on http://example/, or an email address user@example.[27]
ICANN and IAB have spoken out against the practice, classifying it as a security risk among other concerns.[28] ICANN's Security and Stability Advisory Committee (SSAC) additionally claims that SMTP "requires at least two labels in the FQDN of a mail address" and, as such, mail servers would reject emails to addresses with dotless domains.[27]
ICANN has also published a resolution in 2013 that prohibits the creation of dotless domains on gTLDs.[29] ccTLDs, however, fall largely under their respective country's jurisdiction, 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:
Other ccTLDs with A or AAAA records, as of September 2023, include: .cm, .tk and .ws.
A similar query to org's presented above can be made for ai, which shows A and MX records for the TLD:
QUESTIONai. IN ANYANSWERai. 21599 IN A 209.59.119.34ai. 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 the IETF, they were the following:[30] .ac, .dk, .gg, .io, .je, .kh, .sh, .tm, .to, and .vi.
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:[31]
<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 as foo.bar or test.dev, which would both collide with the creation of gTLDs .bar in 2014 and .dev in 2019. As of September 2023, the only top-level domains with these special apex records are .arab and .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 (see IPv4 § Addressing), picked as a mnemonic to indicate a DNS-related problem, as DNS uses port 53.[32]
Several networks, such as BITNET, CSNET, 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, .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, 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.[33]
The anonymity network Tor formerly used the top-level pseudo-domain .onion for onion services, 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. And Namecoin uses the .bit pseudo-domain.
This memo provides some information on the structure of the names in the Domain Name System (DNS), specifically the top-level domain names; and on the administration of domains.
This memo is a policy statement on the implementation of the Domain Style Naming System in the Internet. This memo is an update of RFC-881, and RFC-897. This is an official policy statement of the IAB and the DARPA.