Uniform Resource Name (URN)

A Uniform Resource Name (URN) is a type of URI identifying a resource by a globally unique, persistent name. A URN always uses the urn scheme and provides no information about how or where to access the resource.

Table of Contents

Format

The URN syntax follows this structure:

urn:<NID>:<NSS>

The Namespace Identifier (NID) names the namespace. NIDs are case-insensitive and registered with IANA. The Namespace-Specific String (NSS) identifies the resource within the namespace. The combination of NID and NSS guarantees global uniqueness.

A URN is allowed to carry optional components after the NSS:

  • r-component: resolution parameters, prefixed by ?+
  • q-component: query passed to the resolved resource, prefixed by ?=
  • f-component: fragment identifier, prefixed by #
urn:example:resource?+res-param?=query#fragment

Namespaces

IANA maintains two categories of registered URN namespaces:

Formal namespaces

Formal namespaces serve a broad community of users and go through a structured registration process. The NID is a meaningful string chosen by the registrant. Well-known formal namespaces include:

NID Purpose
isbn International Standard Book Numbers
issn International Standard Serial Numbers
ietf IETF documents and standards
oid Object Identifiers (ISO/ITU-T)
uuid Universally Unique Identifiers

Informal namespaces

Informal namespaces follow a simpler registration process. IANA assigns the NID as the prefix urn- followed by a number (e.g., urn-7). The registrant does not choose the NID. Informal namespaces carry the same rights and responsibilities as formal ones.

The full list of registered namespaces is available in the IANA URN Namespaces registry.

Examples

These examples show the URN scheme identifying resources by name. No location or access method is specified.

A URN identifying an IETF RFC

urn:ietf:rfc:7230

The ietf namespace identifies IETF documents. The NSS rfc:7230 refers to the original HTTP/1.1 Message Syntax and Routing specification, since superseded.

A URN identifying a book by ISBN

urn:isbn:0451450523

The isbn namespace maps to International Standard Book Numbers. This URN persistently identifies the book regardless of which retailer or library holds a copy.

A URN containing a UUID

urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6

The uuid namespace wraps a Universally Unique Identifier, providing a URN for resources identified by UUID.

URNs in practice

HTTP and web standards lean on URNs wherever a stable name matters more than a fetchable address. The urn:ietf:params branch names protocol parameters across specifications: OAuth grant types travel as values like urn:ietf:params:oauth:grant-type:device_code, and WHIP registers its protocol extensions under urn:ietf:params:whip:ext so a server can announce an extension without hosting a page for the identifier. Problem details kept its urn:ietf:rfc:7807 namespace across the specification's revision for the same reason: names already deployed in APIs stay valid.

The uuid namespace does the day-to-day work. WebDAV servers mint lock tokens and resource identifiers as urn:uuid: values, which is why 208 responses carry them, and APIs use the form wherever an identifier needs to be globally unique without a registry. XML-based standards run on URNs as namespace names, with SAML's urn:oasis:names:tc:SAML:2.0:assertion among the most widely deployed.

The pattern across all of these: the identifier appears inside message bodies and protocol fields, never on the wire as a request target, since nothing listens at a name.

Resolution

A URN on its own does not describe how to retrieve the resource. Resolution is the process of translating a URN into one or more URLs or other metadata. Resolution services (such as Handle System, DOI, or application-specific resolvers) map URNs to current resource locations.

Not every URN has a resolver. Many URNs serve purely as stable identifiers within databases, catalogs, and XML namespaces.

See also

Last updated: September 22, 2026