Editing Domain name (section)

==Domain name space==
[[Image:Domain name space.svg|thumb|The hierarchical domain name system, organized into zones, each served by domain name servers]]

Today, the [[Internet Corporation for Assigned Names and Numbers]] (ICANN) manages the top-level development and architecture of the Internet domain name space. It authorizes [[domain name registrar]]s, through which domain names may be registered and reassigned.

[[File:DNS schema.svg|The hierarchy of labels in a fully qualified domain name|thumb|350px]]

The domain name space consists of a [[tree data structure|tree]] of domain names. Each node in the tree holds information associated with the domain name. The tree sub-divides into ''zones'' beginning at the [[DNS root zone]].

===Domain name syntax===
A domain name consists of one or more parts, technically called ''labels'', that are conventionally concatenated, and delimited by dots, such as ''[[example.com]]''.
* The right-most label conveys the [[top-level domain]]; for example, the domain name ''www.example.com'' belongs to the top-level domain ''com''.
* The hierarchy of domains descends from the right to the left label in the name; each label to the left specifies a subdivision, or [[subdomain]] of the domain to the right. For example: the label ''example'' specifies a node ''example.com'' as a subdomain of the ''com'' domain, and ''www'' is a label to create ''www.example.com'', a subdomain of ''example.com''. Each label may contain from 1 to 63 [[octet (computing)|octets]]. The empty label is reserved for the root node and when fully qualified is expressed as the empty label terminated by a [[full stop|dot]]. The full domain name may not exceed a total length of 253 ASCII characters in its textual representation.<ref name="rfc1035">{{cite journal |title=Domain names - Implementation and specification (RFC 1035) |last=Mockapetris |first=P. |date=November 1987 |url=https://datatracker.ietf.org/doc/rfc1035/ |website=IETF Datatracker |access-date=January 21, 2024}}</ref>
* A [[hostname]] is a domain name that has at least one associated IP address. For example, the domain names ''www.example.com'' and ''example.com'' are also hostnames, whereas the ''com'' domain is not. However, other top-level domains, particularly [[country code top-level domain]]s, may indeed have an IP address, and if so, they are also hostnames.
* Hostnames impose restrictions on the characters allowed in the corresponding domain name. A valid hostname is also a valid domain name, but a valid domain name may not necessarily be valid as a hostname.

===Top-level domains===
When the Domain Name System was devised in the 1980s, the domain name space was divided into two main groups of domains.<ref>{{cite web
  | url   = http://www.icann.org/en/tlds/
  | title   = Introduction to Top-Level Domains (gTLDs)
  | publisher   = Internet Corporation for Assigned Names and Numbers (ICANN)
  | access-date   = 2009-06-26
  | archive-date   = 2009-06-15
  | archive-url   = https://web.archive.org/web/20090615180958/http://www.icann.org/en/tlds/
  | url-status   = live
  }}</ref> The [[country code top-level domain]]s (ccTLD) were primarily based on the two-character territory codes of [[ISO 3166-1 alpha-2|ISO-3166]] country abbreviations. In addition, a group of seven [[generic top-level domain]]s (gTLD) was implemented which represented a set of categories of names and multi-organizations.<ref>RFC 920, ''Domain Requirements'', J. Postel, J. Reynolds, The Internet Society (October 1984)</ref> These were the domains [[.gov|gov]], [[.edu|edu]], [[.com|com]], [[.mil|mil]], [[.org|org]], [[.net|net]], and [[.int|int]]. These two types of [[top-level domain]]s (TLDs) are the highest level of domain names of the Internet. Top-level domains form the [[DNS root zone]] of the hierarchical [[Domain Name System]]. Every domain name ends with a top-level domain label.

During the growth of the Internet, it became desirable to create additional generic top-level domains. As of October 2009, 21 generic top-level domains and 250 two-letter country-code top-level domains existed.<ref>[http://www.icann.org/en/topics/new-gtlds/factsheet-new-gtld-program-oct09-en.pdf "New gTLD Program"] {{Webarchive|url=https://web.archive.org/web/20111125093301/http://www.icann.org/en/topics/new-gtlds/factsheet-new-gtld-program-oct09-en.pdf |date=2011-11-25 }}, ICANN, October 2009</ref> In addition, the [[.arpa|ARPA]] domain serves technical purposes in the infrastructure of the Domain Name System.

During the 32nd International Public ICANN Meeting in Paris in 2008,<ref>{{cite web
  | url   = http://par.icann.org/
  | title   = 32nd International Public ICANN Meeting
  | date   = 2008-06-22
  | publisher   = ICANN
  | access-date   = 2009-06-26
  | archive-date   = 2009-03-08
  | archive-url   = https://web.archive.org/web/20090308082309/http://par.icann.org/
  | url-status   = live
  }}</ref> 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 envisions the availability of many new or already proposed domains, as well as a new application and implementation process.<ref>{{cite web
  | url        = http://www.icann.org/en/topics/new-gtld-program.htm
  | title        = New gTLS Program
  | access-date        = 2009-06-15
  | publisher        = ICANN
  | archive-date        = 2011-09-10
  | archive-url        = https://web.archive.org/web/20110910144540/http://www.icann.org/en/topics/new-gtld-program.htm
  | url-status        = live
  }}</ref> Observers believed that the new rules could result in hundreds of new top-level domains to be registered.<ref>[http://www.circleid.com/posts/86269_icann_approves_overhaul_top_level_domains/ ICANN Board Approves Sweeping Overhaul of Top-level Domains] {{Webarchive|url=https://web.archive.org/web/20090626031642/http://www.circleid.com/posts/86269_icann_approves_overhaul_top_level_domains/ |date=2009-06-26 }}, CircleID, 26 June 2008.</ref> In 2012, the program commenced, and received 1930 applications.<ref>{{cite web|url=https://newgtlds.icann.org/en/about/program|title=About the Program - ICANN New gTLDs|publisher=ICANN|access-date=2016-11-09|archive-date=2016-11-03|archive-url=https://web.archive.org/web/20161103092444/https://newgtlds.icann.org/en/about/program|url-status=live}}</ref> By 2016, the milestone of 1000 live gTLD was reached.

The [[Internet Assigned Numbers Authority]] (IANA) maintains an annotated list of top-level domains in the [[DNS root zone]] database.<ref>{{cite web|url=https://www.iana.org/domains/root/db/|title=Root Zone Database|publisher=IANA|access-date=2020-11-01|archive-date=2019-05-04|archive-url=https://web.archive.org/web/20190504123100/https://www.iana.org/domains/root/db|url-status=live}}</ref>

For special purposes, such as network testing, documentation, and other applications, IANA also reserves a set of special-use domain names.<ref name=RFC6761>{{cite journal|last1=Cheshire|first1=S. |last2=Krochmal |first2=M.|title=RFC6761 - Special-Use Domain Names|date=February 2013|url=http://tools.ietf.org/html/rfc6761|website=Internet Engineering Task Force |doi=10.17487/RFC6761|access-date=3 May 2015|archive-date=13 November 2020|archive-url=https://web.archive.org/web/20201113073142/https://tools.ietf.org/html/rfc6761|url-status=live|url-access=subscription}}</ref> This list contains domain names such as [[.example|example]], [[.local|local]], [[localhost]], and [[.test|test]]. Other top-level domain names containing trade marks are registered for corporate use. Cases include brands such as [[BMW]], [[Google]], and [[Canon Inc.|Canon]].<ref>{{cite web|url=https://observatory.domains/observatory/executive-summary/|title=Executive Summary - dot brand observatory|publisher=observatory.domains|access-date=2016-11-09|archive-date=2016-11-10|archive-url=https://web.archive.org/web/20161110043740/https://observatory.domains/observatory/executive-summary/|url-status=dead}}</ref>

===Second-level and lower level domains===
Below the top-level domains in the domain name hierarchy are the [[second-level domain]] (SLD) names. These are the names directly to the left of .com, .net, and the other top-level domains. As an example, in the domain ''example.co.uk'', ''co'' is the second-level domain.

Next are third-level domains, which are written immediately to the left of a second-level domain. There can be fourth- and fifth-level domains, and so on, with virtually no limitation. Each label is separated by a [[full stop]] (dot). An example of an operational domain name with four levels of domain labels is ''sos.state.oh.us''. 'sos' is said to be a sub-domain of 'state.oh.us', and 'state' a sub-domain of 'oh.us', etc. In general, [[subdomain]]s are domains subordinate to their parent domain. An example of very deep levels of subdomain ordering are the [[IPv6]] reverse resolution [[DNS zone]]s, e.g., 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa, which is the reverse DNS resolution domain name for the IP address of a [[loopback]] interface, or the [[localhost]] name.

Second-level (or lower-level, depending on the established parent hierarchy) domain names are often created based on the name of a company (e.g., ''bbc''.co.uk), product or service (e.g. ''hotmail''.com). Below these levels, the next domain name component has been used to designate a particular host server. Therefore, ''ftp.example.com'' might be an FTP server, ''www.example.com'' would be a [[World Wide Web]] server, and ''mail.example.com'' could be an email server, each intended to perform only the implied function. Modern technology allows multiple physical servers with either different (cf. [[Load balancing (computing)|load balancing]]) or even identical addresses (cf. [[anycast]]) to serve a single hostname or domain name, or multiple domain names to be served by a single computer. The latter is very popular in [[Web hosting service]] centers, where service providers host the websites of many organizations on just a few servers.

The hierarchical [[DNS label]]s or components of domain names are separated in a fully qualified name by the [[full stop]] (dot, ''.'').

===Internationalized domain names===
{{Main|Internationalized domain name}}
The character set allowed in the Domain Name System is based on [[ASCII]] and does not allow the representation of names and words of many languages in their native scripts or alphabets. [[ICANN]] approved the [[Internationalized domain name]] (IDNA) system, which maps [[Unicode]] strings used in application user interfaces into the valid DNS character set by an encoding called [[Punycode]]. For example, københavn.eu is mapped to xn--kbenhavn-54a.eu. Many [[domain name registry|registries]] have adopted IDNA.