Uniform Resource Identifier

This Euler diagram shows that a uniform resource identifier (URI) is either a Uniform resource locator (English URLs), as well as a uniform resource name (URN, English acronym), or both at the same time.

A uniform resource identifier or URI —from English uniform resource identifier— is a string of characters that identifies resources –physical or abstract – of a network in a unique way. The difference with respect to a uniform resource locator (URL) is that the latter refer to resources that, in general, can vary over time.

Normally these resources are accessible on a network or system. URIs can be Uniform Resource Locators (URLs), Uniform Resource Names (URNs), or both.

A URI consists of the following parts:

• Scheme: name that refers to a specification to assign identifiers, e.g. urn:, tag:, cid:. In some cases it also identifies the access protocol to the resource, for example http:, mailto:, ftp:etc.
• Authority: hierarchical element that identifies the name authority (e.g. //www.example.com).
• Route: Information usually organized in hierarchical form, which identifies the resource in the scope of the URI scheme and the authority of names (e.g. /domains/example).
• Consultation: Information with non- hierarchical structure (usually pairs "key=value") that identifies the resource within the URI scheme and the name authority. The beginning of this component is indicated by the '?' character.
• Fragment: Allows to identify a part of the main resource, or view of a representation thereof. The beginning of this component is indicated by the '#' character.

Although it is customary to call all web addresses URLs, URI is a more complete identifier and that is why its use is recommended instead of the URL expression.

A URI differs from a URL in that it allows a subaddress to be included in the address, determined by the “fragment”.

When the authority is divided into the form of user, host and port, this can be represented by the following syntax diagram: