RFC1630 (June 1994, Universal Resource Identifiers in WWW) introduces three fundamental concepts:
A general-purpose namespace mechanism
An instance of a URI that is the address of some resource, accessible by means of a protocol such as HTTP or NNTP
An instance of a URI that, unlike a fragile URL, is guaranteed to remain always available
RFC1736 (February 1995, Functional Recommendations for Internet Resource Locators) recommends that URL schemes should be:
Global in scope
Human readable and writable
RFC1738 (December 1994, Uniform Resource Locators (URL)) describes common URL schemes, including http:, news:, mailto:, and file:. RFC1808 ( June 1995, Relative Uniform Resource Locators) defines the rules for relative URLs that are fully qualified with reference to a stated or implied base URL.
RFC2396 (August 1998, Uniform Resource Identifiers (URI): Generic Syntax) subsumes RFC1738 and RFC1808, defining “a single generic syntax for all URI.” A URI, say the authors, “can be further classifed as a name, a locator, or both.” It reiterates RFC1630’s URI/URL/URN taxonomy: “A URN differs from a URL in that its primary purpose is persistent labeling of a resource with an identifier.”
The URL/URN distinction can be misleading. A World Wide Web Consortium (W3C) document entitled “Cool URIs don’t change” (http://www.w3.org/Provider/Style/URI) notes astutely: ...