Overview#Uniform Resource Name (URN) is the historical name for a Uniform Resource Identifier (URI) that uses the urn scheme.
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).
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.
Syntax#The syntax of a URN is represented in BNF form as:
<URN> ::= "urn:" <NID> ":" <NSS>
- 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#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#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
Experimental#Experimental namespaces take the form
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?
More Information#There might be more information for this subject on one of the following:
- Namespace Identifier
- RFC 1737
- RFC 2141
- RFC 3406
- RFC 3615
- Society for Worldwide Interbank Financial Telecommunication
- URIs, URLs, and URNs
- Uniform Resource Identifier
- Unique Identifier