URn

URN, or Uniform Resource Name, is a type of Uniform Resource Identifier (URI) designed for persistent naming of resources. Unlike URLs, which identify a resource by its current location, URNs aim to remain valid even if the resource moves or changes location. URIs encompass both URLs and URNs, providing a framework for identifying resources without prescribing their retrieval mechanism.

The format of a URN is urn:<NID>:<NSS>. The NID, or namespace identifier, specifies the namespace under which the resource is identified, and the NSS, or namespace-specific string, is defined by that namespace. NIDs are registered with IANA and described in RFCs. Common examples include urn:isbn:9780306406157, urn:issn:1234-5679, urn:ietf:rfc:3986, and urn:doi:10.1000/xyz123. The exact rules for the NSS depend on the particular NID.

Resolution and use vary by namespace. A URN may be resolvable through a URN resolver if a corresponding registry exists; otherwise, it functions primarily as a persistent identifier. In practice, namespaces such as isbn and issn have established registries and resolution services, while others (such as doi or ietf) are tied to specific systems for locating or identifying the resource.

Standards and history: The concept originated in the IETF and was formalized in RFC 8141, which updates the syntax, semantics, and registration process for URN namespaces. RFC 2141 previously defined core URI syntax related to URNs. The URN framework is intended to complement URLs by emphasizing stable, location-independent naming for long-lived resources.