Editing Domain Name System (section)

===Domain name syntax, internationalization===
The definitive descriptions of the rules for forming domain names appear in RFC 1035, RFC 1123, RFC 2181, and RFC 5892.  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 right to left; each label to the left specifies a subdivision, or [[subdomain]] of the domain to the right. For example, the label ''example'' specifies a subdomain of the ''com'' domain, and ''www'' is a subdomain of example.com. This tree of subdivisions may have up to 127 levels.<ref>{{Cite book|title=International Domain Name Law: ICANN and the UDRP|last=Lindsay|first=David|publisher=Bloomsbury Publishing|year=2007|isbn=978-1-84113-584-7|pages=8}}</ref>

A label may contain zero to 63 characters, because the length is only allowed to take 6 bits. The null label of length zero is reserved for the root zone. The full domain name may not exceed the length of 253 characters in its textual representation (or 254 with the trailing dot).<ref name=rfc1034/> In the internal binary representation of the DNS this maximum length of 253 requires 255 octets of storage, as it also stores the length of the first of many labels and adds last null byte.<ref name=rfc1035/> 255 length is only achieved with at least 6 labels (counting the last null label).{{Citation needed|date=November 2024}}

Although no technical limitation exists to prevent domain name labels from using any character that is representable by an octet, hostnames use a preferred format and character set. The characters allowed in labels are a subset of the [[ASCII]] character set, consisting of characters ''a'' through ''z'', ''A'' through ''Z'', digits ''0'' through ''9'', and hyphen. This rule is known as the ''LDH rule'' (letters, digits, hyphen). Domain names are interpreted in a case-independent manner.{{Ref RFC|4343}} Labels may not start or end with a hyphen.{{Ref RFC|3696}} An additional rule requires that top-level domain names should not be all-numeric.<ref name=rfc3696 />

The limited set of ASCII characters permitted in the DNS prevented the representation of names and words of many languages in their native alphabets or scripts. To make this possible, [[ICANN]] approved the [[Internationalized domain name|Internationalizing Domain Names in Applications]] (IDNA) system, by which user applications, such as web browsers, map [[Unicode]] strings into the valid DNS character set using [[Punycode]]. In 2009, ICANN approved the installation of internationalized domain name [[Country code top-level domain|country code top-level domains (''ccTLD''s)]]. In addition, many [[domain name registry|registries]] of the existing top-level domain names ([[Top-level domain|''TLD'']]s) have adopted the IDNA system, guided by RFC 5890, RFC 5891, RFC 5892, RFC 5893.