URNs were originally conceived to be part of a three-part
information architecture for the Internet, along with
Uniform Resource Locators (URLs) and
Uniform Resource Characteristics (URCs), a
metadata framework. As described in , and later in , URNs were distinguished from URLs, which identify resources by specifying their locations in the context of a particular access protocol, such as
HTTP or
FTP. In contrast, URNs were conceived as
persistent, location-independent identifiers assigned within defined
namespaces, typically by an authority responsible for the namespace, so that they are globally unique and persistent over long periods of time, even after the resource which they identify ceases to exist or becomes unavailable. URCs never progressed past the conceptual stage, and other technologies such as the
Resource Description Framework later took their place. Since in 2005, use of the terms "Uniform Resource Name" and "Uniform Resource Locator" has been deprecated in technical standards in favor of the term Uniform Resource Identifier (URI), which encompasses both, a view proposed in 2001 by a joint working group between the
World Wide Web Consortium (W3C) and
Internet Engineering Task Force (IETF). A URI is a
string of
characters used to
identify or
name a
resource on the internet. URIs are used in many Internet protocols to refer to and access information resources. URI schemes include the http and ftp protocols, as well as hundreds of others. In the "contemporary view", as it is called, all URIs identify or name resources, perhaps uniquely and persistently, with some of them also being "locators" which are resolvable in conjunction with a specified protocol to a representation of the resources. Other URIs are not locators and are not necessarily resolvable within the bounds of the systems where they are found. These URIs may serve as names or identifiers of resources. Since resources can move, opaque identifiers which
are not locators and are not bound to particular locations are arguably more likely than identifiers which
are locators to remain unique and persistent over time. But whether a URI is resolvable depends on many operational and practical details, irrespective of whether it is called a "name" or a "locator". In the contemporary view, there is no bright line between "names" and "locators". In accord with this way of thinking, the distinction between Uniform Resource
Names and Uniform Resource
Locators is now no longer used in formal
Internet Engineering Task Force technical standards, though the latter term, URL, is still in wide informal use. The term "URN" continues now as one of more than a hundred URI "schemes", urn:, paralleling http:, ftp:, and so forth. URIs of the urn: scheme are not locators, are not required to be associated with a particular protocol or access method, and need not be resolvable. They should be assigned by a procedure which provides some assurance that they will remain unique and identify the same resource persistently over a prolonged period. Some namespaces under the urn: scheme, such as
urn:uuid: assign identifiers in a manner which does not require a registration authority, but most of them do. A typical URN namespace is
urn:isbn, for International Standard Book Numbers. This view is continued in (2017). There are other URI schemes, such as
tag:,
info: (now largely deprecated), and ni: which are similar to the urn: scheme in not being locators and not being associated with particular resolution or access protocols. ==Syntax==