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A Uniform Resource Identifier (or URI) is a string of characters used to identify a name or resource. URI is legacy terminology. In most cases, the term URL should be used instead.

A Uniform Resource Identifier (or URI) is a string of characters used to identify a name or resource.

Note: The current URL Standard at https://url.spec.whatwg.org/#url-representation supersedes RFC 3986 and dispenses with the use of the term URI, as documented in its Goals section:

Standardize on the term URL. URI and IRI are just confusing. In practice a single algorithm is used for both so keeping them distinct is not helping anyone. URL also easily wins the search result popularity contest.

But the information below documents URI in terms of the legacy RFC 3986 spec.

Structure

A URI can be a Uniform Resource Locator url, a Uniform Resource Name urn, or both.

RFC 3986 defines a URI as composed of the following parts:

    foo://example.com:8042/over/there?name=ferret#nose
    \_/   \______________/\_________/ \_________/ \__/
     |           |            |            |        |
  scheme     authority       path        query   fragment

Then defining relative URIs ( Section 5.2), you can omit any of those sections, always starting from the left. In pseudo-code, it looks like this:

 result = ""

  if defined(scheme) then
     append scheme to result;
     append ":" to result;
  endif;

  if defined(authority) then
     append "//" to result;
     append authority to result;
  endif;

  append path to result;

  if defined(query) then
     append "?" to result;
     append query to result;
  endif;

  if defined(fragment) then
     append "#" to result;
     append fragment to result;
  endif;

  return result;

The URI you are describing is a scheme-less relative URI.

Examples

For example, in the following URI:

https://stackru.com/questions/tagged/uri?sort=newest&pagesize=50#questions

... the components are:

  • scheme: https

  • hierarchical part: //stackru.com/questions/tagged/uri

  • query: sort=newest&pagesize=50

  • fragment: questions

The precise syntax of the individual components varies depending on the scheme. Here are some examples:

  • mailto:nobody@example.com?subject=Hello&body=Is%20anybody%20home%3F

  • data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=

  • urn:ietf:rfc:3986

URL

A URL is a URI but is more specific. In addition to identifying a web resource, a URL specifies the means of acting upon or obtaining the representation, specifying both its primary access mechanism and network location.

For example, the URL http://example.org/wiki/Main_Page refers to a resource identified as /wiki/Main_Page whose representation, in the form of HTML and related code, obtainable via HyperText Transfer Protocol (http) from a network host whose domain name is example.org.

URI Reference

A URI reference may take the form of a full URI, or just the scheme-specific portion of one, or even some trailing component thereof – even the empty string. An optional fragment identifier, preceded by #, may be present at the end of a URI reference. The part of the reference before the # indirectly identifies a resource, and the fragment identifier identifies some portion of that resource.

To derive a URI from a URI reference, software converts the URI reference to 'absolute' form by merging it with an absolute 'base' URI according to a fixed algorithm. The system treats the URI reference as relative to the base URI, although in the case of an absolute reference, the base has no relevance. The base URI typically identifies the document containing the URI reference, although this can be overridden by declarations made within the document or as part of an external data transmission protocol. If the base URI includes a fragment identifier, it is ignored during the merging process. If a fragment identifier is present in the URI reference, it is preserved during the merging process.

Web document markup languages frequently use URI references to point to other resources, such as external documents or specific portions of the same logical document.

Uses of URI references in markup languages

  • In HTML, the value of the src attribute of the img element provides a URI reference, as does the value of the href attribute of the a or link element.
  • In XML, the system identifier appearing after the SYSTEM keyword in a DTD is a fragmentless URI reference.
  • In XSLT, the value of the href attribute of the xsl:import element/instruction is a URI reference; likewise the first argument to the document() function.

Examples of absolute URIs

Examples of URI references