Uniform Resource Name (URN) is the historical name for a Uniform Resource Identifier (URI) that uses the urn scheme.

Defined in 1997 in RFC 2141, Uniform Resource Names were intended to serve as persistent, location-independent identifiers, allowing the simple mapping of namespaces into a single URN namespace.

The existence of such a URI does not imply availability of the identified resource, but such URIs are required to remain globally unique and persistent, even when the resource ceases to exist or becomes unavailable. RFC 3986

Since RFC 3986 in 2005, the use of the term has been deprecated in favor of the less-restrictive "URI", a view proposed by a joint working group between the World Wide Web Consortium (W3C) and Internet Engineering Task Force (IETF).

Both URNs and Uniform Resource Locators (URLs) are URIs, and a particular URI may be a name as well as a locator at the same time.

URNs were originally intended in the 1990s to be part of a three-part information architecture for the Internet, along with URLs and Uniform Resource Characteristics (URCs), a metadata framework. However, URCs never progressed past the conceptual stage, and other technologies such as the Resource Description Framework later took their place.


The syntax of a URN is represented in BNF form as:
<URN> ::= "urn:" <NID> ":" <NSS>
This renders as:
  • urn:<NID>:<NSS> - The leading urn: sequence is case-insensitive. <NID> is the Namespace Identifier, which determines the syntactic interpretation of <NSS>, the namespace-specific string. The functional requirements for Uniform Resource Names are described in RFC 1737.
  • Namespaces - In order to ensure the global uniqueness of URN namespaces, their identifiers (NIDs) are required to be registered with the IANA. Registered namespaces may be "formal" or "informal". An exception to the registration requirement is made for "experimental namespaces"


Formal namespaces are those where some Internet users are expected to benefit from their publication,(RFC 3406) and are subject to several restrictions. They must:
  • not be an already-registered NID
  • not start with x- (see "Experimental namespaces", below)
  • not start with urn-
  • not start with XY-, where XY is any combination of two ASCII letters
  • be more than two letters long.


Informal namespaces are registered with IANA and assigned a number sequence (chosen by IANA on a first-come-first-served basis) as an identifier, in the format
"urn-" <number>
Informal namespaces are fully fledged URN namespaces and can be registered in global registration services.


Experimental namespaces take the form
"X-" <NID>
Namespaces of this form are intended only for use within internal or limited experimental contexts, and are not required or expected to be globally unique.

RFC 6648 deprecated the use of "X-" notation for new ID names, but makes no recommendation on substituting existing "X-" names, and does not override existing specifications that require the use of "X-".

URIs, URLs, and URNs#

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« This page (revision-9) was last changed on 01-Dec-2016 13:07 by jim