Uniform Resource Identifier

URI, short for Uniform Resource Identifier, is an Internet protocol created by Tim-Berners Lee. It was codified by the IETF as RFC 2396.

A URI is a short string of characters that conform to a certain syntax. The string indicates a name or address that can be used to refer to an abstract or physical resource.

URLs, Uniform Resource Locators, are the most common type of URI.

URNs[?], Uniform Resource Names, are another type of URI.

The URI syntax is essentially a URI scheme name like "http", "ftp", "mailto", "urn", etc., followed by a colon character, and then a scheme-specific part. The semantics of the scheme-specific part are determined by the specifications that govern the schemes, although the URI syntax does force all schemes to reserve certain characters for special purposes, without always saying what those purposes are.

URI References and URI Resolution

Some Internet document formats such as HTML and XML require URIs within a document to be written in the form of a URI reference. A URI reference is a special type of URI that is either an "absolute", complete URI, or a "relative", partial URI having no scheme name and colon. A URI reference can even be an empty string.

The document itself has a URI to identify it, and the URI reference, if relative, is considered to be relative to that URI, except in cases when the document or some out-of-band means has been used to establish a different base URI.

To "resolve" a URI means either to convert a relative URI reference to absolute form, or to dereference a URI or URI reference by attempting to obtain a representation of the resource that it identifies. The "resolver" component in document processing software generally provides both services.

If a URI reference, when resolved to absolute form, is determined to be equivalent to the URI of the document containing the reference, the document processing software is supposed to use its current representation of the document to satisfy the resolution; a new representation is not to be fetched.

A URI reference may also have, appended to the end, a fragment identifier, immediately preceded by "#". The fragment identifier is ignored until the very end of the resolution process.