Editing Uniform Resource Name (section)

==Namespaces==
In order to ensure the global uniqueness of URN namespaces, their identifiers (NIDs) are required to be registered with the [[Internet Assigned Numbers Authority|IANA]]. Registered namespaces may be "formal" or "informal". An exception to the registration requirement was formerly made for "experimental namespaces",{{sfnp|RFC 3406|2002}} since rescinded by <nowiki>RFC</nowiki> 8141.{{sfnp|RFC 8141|2017}}

===Formal===
Approximately sixty formal URN namespace identifiers have been registered. These are namespaces where Internet users are expected to benefit from their publication,{{sfnp|RFC 8141|2017}} and are subject to several restrictions. They must:

* Not be an already-registered NID
* Not start with {{code|urn-}}
* Be more than two letters long
* Not start with {{code|XY-}}, where XY is any combination of two [[ASCII]] letters
* Not start with {{code|x-}} (see "Experimental namespaces", below)

===Informal===
Informal namespaces are registered with IANA and assigned a number sequence (chosen by IANA on a first-come-first-served basis) as an identifier,{{sfnp|RFC 8141|2017}} in the format
:<code>"urn-" {{angbr|number}}</code>

Informal namespaces are fully fledged URN namespaces and can be registered in global registration services.{{sfnp|RFC 8141|2017}}

===Experimental===
An exception to the registration requirement was formerly made for "experimental namespaces".{{sfnp|RFC 3406|2002}} However, following the deprecation of the "X-" notation for new identifier names,{{sfnp|RFC 6648|2012}} {{IETF RFC|8141|link=no}}{{sfnp|RFC 8141|2017}} did away with experimental URN namespaces, indicating a preference for use of the <code>urn:example</code> namespace where appropriate.{{sfnp|RFC 6963|2013}}