To paraphrase the World Wide Web Consortium, Internet space is inhabited by many points of content. A URI (Uniform Resource Identifier; pronounced YEW-AHR-EYE) is the way you identify any of those points of content, whether it be a page of text, a video or sound clip, a still or animated image, or a program. The most common form of URI is the Web page address, which is a particular form or subset of URI called a Uniform Resource Locator (URL). A URI typically describes:
- The mechanism used to access the resource
- The specific computer that the resource is housed in
- The specific name of the resource (a file name) on the computer
For example, this URI:
identifies a file that can be accessed using the Web protocol application, Hypertext Transfer Protocol, ("http://") that is housed on a computer named "www.w3.org" (which can be mapped to a unique Internet address). In the computer's directory structure, the file is located at the pathname of "/Icons/WWW/w3c_main.gif." Character strings that identify File Transfer Protocol FTP addresses and e-mail addresses are also URIs (and, like the HTTP address, are also the specific subset of URI called a URL).
Another kind of URI is the Uniform Resource Name (URN). A URN is a form of URI that has "institutional persistence," which means that its exact location may change from time to time, but some agency will be able to find it.
The URI rules of syntax, set forth in the Internet Engineering Task Force (IETF) Request for Comments 1630, apply for all Internet addresses. In Tim Berner-Lee's original working document, URI stood for Universal Resource Identifier.