Top-level domain
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.[9] 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 ICANN, an Internet multi-stakeholder community, which operates the Internet Assigned Numbers Authority (IANA), and is in charge of maintaining the DNS root zone.
"TLD" and "TLDN" redirect here. For Temporary Location Directory Number, see Mobile Station Roaming Number. For other uses, see TLD (disambiguation).History[edit]
Originally, the top-level domain space was organized into three main groups: Countries, Categories, and Multiorganizations.[10] An additional temporary group consisted of only the initial DNS domain, .arpa,[11] and was intended for transitional purposes toward the stabilization of the domain name system.
As of 2015, IANA distinguishes the following groups of top-level domains:[12]
Countries are designated in the Domain Name System by their two-letter ISO country code;[13] there are exceptions, however (e.g., .uk). This group of domains is, therefore, commonly known as country-code top-level domains (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, .edu, .com, .mil, .org, 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 at https://www.iana.org/domains/root/db/.
Internationalized country code TLDs[edit]
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 characters). 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,[14] 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.[15]
Infrastructure domain[edit]
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.[16]
A set of domain names is reserved[17][18] 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.[19] 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.[20]
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.[21]
ICANN proposed the use of .internal on 2024-01-24[22] with request for feedback. In 2017 an Internet-Draft draft-wkumari-dnsop-internal-00 already proposed 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.