Uniform Resource Identifier

Source: Wikipedia, the free encyclopedia.
Uniform Resource Identifier (URI)
DomainWorld Wide Web

A Uniform Resource Identifier (URI) is a unique sequence of characters that identifies a logical or physical resource used by web technologies. URIs may be used to identify anything, including real-world objects, such as people and places, concepts, or information resources such as web pages and books. Some URIs provide a means of locating and retrieving information resources on a network (either on the Internet or on another private network, such as a computer filesystem or an

Friend of a Friend vocabulary
would each have an individual URI.



URIs and URLs have a shared history. In 1990, Tim Berners-Lee's proposals for hypertext implicitly introduced the idea of a URL as a short string representing a resource that is the target of a hyperlink.[1] At the time, people referred to it as a "hypertext name"[2] or "document name".

Over the next three and a half years, as the World Wide Web's core technologies of HTML, HTTP, and web browsers developed, a need to distinguish a string that provided an address for a resource from a string that merely named a resource emerged. Although not yet formally defined, the term Uniform Resource Locator came to represent the former, and the more contentious Uniform Resource Name came to represent the latter. In July 1992 Berners-Lee's report on the

BOF" mentions URLs (as Uniform Resource Locators), URNs (originally, as Unique Resource Numbers), and the need to charter a new working group.[3] In November 1992 the IETF "URI Working Group" met for the first time.[4]

During the debate over defining URLs and URNs, it became evident that the concepts embodied by the two terms were merely aspects of the fundamental, overarching, notion of resource identification. In June 1994, the

RFC 1630 attempted to summarize the syntaxes of URL schemes in use at the time. It acknowledged -- but did not standardize -- the existence of relative URLs and fragment identifiers.[5]


In December 1994,

RFC 1738 formally defined relative and absolute URLs, refined the general URL syntax, defined how to resolve relative URLs to absolute form, and better enumerated the URL schemes then in use.[6] The agreed definition and syntax of URNs had to wait until the publication of IETF RFC 2141[7]
in May 1997.

The publication of IETF RFC 2396

. The new RFC changed the meaning of "U" in "URI" to "Uniform" from "Universal".

In December 1999,

RFC 2732[10] provided a minor update to RFC 2396, allowing URIs to accommodate IPv6 addresses. A number of shortcomings discovered in the two specifications led to a community effort, coordinated by RFC 2396 co-author Roy Fielding, that culminated in the publication of IETF RFC 3986[11] in January 2005. While obsoleting the prior standard, it did not render the details of existing URL schemes obsolete; RFC 1738 continues to govern such schemes except where otherwise superseded. IETF RFC 2616[12]
for example, refines the http scheme. Simultaneously, the IETF published the content of RFC 3986 as the full standard STD 66, reflecting the establishment of the URI generic syntax as an official Internet protocol.

In 2001, the W3C's Technical Architecture Group (TAG) published a guide to

best practices and canonical URIs for publishing multiple versions of a given resource.[13]
For example, content might differ by language or by size to adjust for capacity or settings of the device used to access that content.

In August 2002, IETF

RFC 3305[14] pointed out that the term "URL" had, despite widespread public use, faded into near obsolescence, and serves only as a reminder that some URIs act as addresses by having schemes implying network accessibility, regardless of any such actual use. As URI-based standards such as Resource Description Framework
make evident, resource identification need not suggest the retrieval of resource representations over the Internet, nor need they imply network-based resources at all.

The Semantic Web uses the HTTP URI scheme to identify both documents and concepts in the real world, a distinction which has caused confusion as to how to distinguish the two. The TAG published an e-mail in 2005 on how to solve the problem, which became known as the httpRange-14 resolution.[15] The W3C subsequently published an Interest Group Note titled Cool URIs for the Semantic Web, which explained the use of content negotiation and the HTTP 303 response code for redirections in more detail.[16]


URLs and URNs


International Standard Book Number (ISBN) system, ISBN 0-486-27557-4 identifies a specific edition of Shakespeare's play Romeo and Juliet
. The URN for that edition would be urn:isbn:0-486-27557-4. However, it gives no information as to where to find a copy of that book.


Hypertext Transfer Protocol (http:) from a network host whose domain name is example.org. (In this case, HTTP usually implies it to be in the form of HTML
and related code. In practice, that is not necessarily the case, as HTTP allows specifying arbitrary formats in its header.)

A URN is analogous to a person's name, while a URL is analogous to their street address. In other words, a URN identifies an item and a URL provides a method for finding it.

Technical publications, especially standards produced by the

W3C Recommendation of 30 July 2001, which acknowledges the precedence of the term URI rather than endorsing any formal subdivision into URL and URN.

URL is a useful but informal concept: a URL is a type of URI that identifies a resource via a representation of its primary access mechanism (e.g., its network "location"), rather than by some other attributes it may have.[17]

As such, a URL is simply a URI that happens to point to a resource over a network.[a][18] However, in non-technical contexts and in software for the World Wide Web, the term "URL" remains widely used. Additionally, the term "web address" (which has no formal definition) often occurs in non-technical publications as a synonym for a URI that uses the http or https schemes. Such assumptions can lead to confusion, for example, in the case of XML namespaces that have a visual similarity to resolvable URIs.

Specifications produced by the WHATWG prefer URL over URI, and so newer HTML5 APIs use URL over URI.[19]

Standardize on the term URL. URI and IRI [Internationalized Resource Identifier] 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.[20]

While most URI schemes were originally designed to be used with a particular

protocol, and often have the same name, they are semantically different from protocols. For example, the scheme http is generally used for interacting with web resources using HTTP, but the scheme file
has no protocol.


A URI has a scheme that refers to a specification for assigning identifiers within that scheme. As such, the URI syntax is a federated and extensible naming system wherein each scheme's specification may further restrict the syntax and semantics of identifiers using that scheme. The URI generic syntax is a superset of the syntax of all URI schemes. It was first defined in

RFC 3986, published in January 2005.[21]

A URI is composed from an allowed set of ASCII characters consisting of reserved characters (generic: :, /, ?, #, [, ], and @; scheme- or implementation-specific: !, $, &, ', (, ), *, +, ,, ;, and =),[22] unreserved characters (uppercase and lowercase letters, decimal digits, -, ., _, and ~),[23] and the character %.[24] Syntax components and subcomponents are separated by delimiters from the reserved characters (only from generic reserved characters for components) and define identifying data represented as unreserved characters, reserved characters that do not act as delimiters in the component and subcomponent respectively,[25] and percent-encodings when the corresponding character is outside the allowed set or is being used as a delimiter of, or within, the component. A percent-encoding of an identifying data octet is a sequence of three characters, consisting of the character % followed by the two hexadecimal digits representing that octet's numeric value.[24]

The URI generic syntax consists of five components organized hierarchically in order of decreasing significance from left to right:[26]

URI = scheme ":" ["//" authority] path ["?" query] ["#" fragment]

A component is undefined if it has an associated delimiter and the delimiter does not appear in the URI; the scheme and path components are always defined.[27] A component is empty if it has no characters; the scheme component is always non-empty.[26]

The authority component consists of subcomponents:

authority = [userinfo "@"] host [":" port]

This is represented in a syntax diagram as:

URI syntax diagram

The URI comprises:

By convention, in http and https URIs, the last part of a path is named pathinfo and it is optional. It is composed by zero or more path segments that do not refer to an existing physical resource name (e.g. a file, an internal module program or an executable program) but to a logical part (e.g. a command or a qualifier part) that has to be passed separately to the first part of the path that identifies an executable module or program managed by a web server; this is often used to select dynamic content (a document, etc.) or to tailor it as requested (see also: CGI and PATH_INFO, etc.).
URI: "http://www.example.com/questions/3456/my-document"
where: "/questions" is the first part of the path (an executable module or program) and "/3456/my-document" is the second part of the path named pathinfo, which is passed to the executable module or program named "/questions" to select the requested document.
An http or https URI containing a pathinfo part without a query part may also be referred to as a 'clean URL' whose last part may be a 'slug'.
Query delimiter Example
Ampersand (&) key1=value1&key2=value2
Semicolon (;)[d] key1=value1;key2=value2

The scheme- or implementation-specific reserved character + may be used in the scheme, userinfo, host, path, query, and fragment, and the scheme- or implementation-specific reserved characters !, $, &, ', (, ), *, ,, ;, and = may be used in the userinfo, host, path, query, and fragment. Additionally, the generic reserved character : may be used in the userinfo, path, query and fragment, the generic reserved characters @ and / may be used in the path, query and fragment, and the generic reserved character ? may be used in the query and fragment.[33]

Example URIs

The following figure displays example URIs and their component parts.

          userinfo       host      port
          ┌──┴───┐ ┌──────┴──────┐ ┌┴┐
  https://[email protected]:123/forum/questions/?tag=networking&order=newest#top
  └─┬─┘   └─────────────┬────────────┘└───────┬───────┘ └────────────┬────────────┘ └┬┘
  scheme          authority                  path                  query           fragment

  └┬─┘   └─────┬─────┘└─┬─┘ └──────┬──────┘
  scheme   authority   path      query

  mailto:[email protected]
  └─┬──┘ └────┬─────────────┘
  scheme     path

  └┬─┘ └─────────────┬─────────────────┘
  scheme            path

  └┬┘ └──────┬──────┘
  scheme    path

  └─┬──┘   └─────┬─────┘
  scheme     authority  path

  └┬┘ └──────────────────────┬──────────────────────┘
  scheme                    path

DOIs (digital object identifiers) fit within the Handle System and fit within the URI system, as facilitated by appropriate syntax.

URI references

A URI reference is either a URI or a relative reference when it does not begin with a scheme component followed by a colon (:).[34] A path segment that contains a colon character (e.g., foo:bar) cannot be used as the first path segment of a relative reference if its path component does not begin with a slash (/), as it would be mistaken for a scheme component. Such a path segment must be preceded by a dot path segment (e.g., ./foo:bar).[35]

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:[36]

  • 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
    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.


Resolving a URI reference against a base URI results in a target URI. This implies that the base URI exists and is an absolute URI (a URI with no fragment component). The base URI can be obtained, in order of precedence, from:[37]

  • the reference URI itself if it is a URI;
  • the content of the representation;
  • the entity encapsulating the representation;
  • the URI used for the actual retrieval of the representation;
  • the context of the application.

Within a representation with a well defined base URI of


a relative reference is resolved to its target URI as follows:[38]

"g:h"     -> "g:h"
"g"       -> "http://a/b/c/g"
"./g"     -> "http://a/b/c/g"
"g/"      -> "http://a/b/c/g/"
"/g"      -> "http://a/g"
"//g"     -> "http://g"
"?y"      -> "http://a/b/c/d;p?y"
"g?y"     -> "http://a/b/c/g?y"
"#s"      -> "http://a/b/c/d;p?q#s"
"g#s"     -> "http://a/b/c/g#s"
"g?y#s"   -> "http://a/b/c/g?y#s"
";x"      -> "http://a/b/c/;x"
"g;x"     -> "http://a/b/c/g;x"
"g;x?y#s" -> "http://a/b/c/g;x?y#s"
""        -> "http://a/b/c/d;p?q"
"."       -> "http://a/b/c/"
"./"      -> "http://a/b/c/"
".."      -> "http://a/b/"
"../"     -> "http://a/b/"
"../g"    -> "http://a/b/g"
"../.."   -> "http://a/"
"../../"  -> "http://a/"
"../../g" -> "http://a/g"

URL munging

URL munging is a technique by which a command is appended to a URL, usually at the end, after a "?" token. It is commonly used in WebDAV as a mechanism of adding functionality to HTTP. In a versioning system, for example, to add a "checkout" command to a URL, it's written as http://editing.com/resource/file.php?command=checkout. It has the advantage of both being easy for CGI parsers and also acts as an intermediary between HTTP and underlying resource, in this case.[39]

Relation to XML namespaces

In XML, a namespace is an abstract domain to which a collection of element and attribute names can be assigned. The namespace name is a character string which must adhere to the generic URI syntax.[40] However, the name is generally not considered to be a URI,[41] because the URI specification bases the decision not only on lexical components, but also on their intended use. A namespace name does not necessarily imply any of the semantics of URI schemes; for example, a namespace name beginning with http: may have no connotation to the use of the HTTP.

Originally, the namespace name could match the syntax of any non-empty URI reference, but the use of relative URI references was deprecated by the W3C.[42] A separate W3C specification for namespaces in XML 1.1 permits Internationalized Resource Identifier (IRI) references to serve as the basis for namespace names in addition to URI references.[43]

See also


  1. ^ A report published in 2002 by a joint W3C/IETF working group aimed to normalize the divergent views held within the IETF and W3C over the relationship between the various 'UR*' terms and standards. While not published as a full standard by either organization, it has become the basis for the above common understanding and has informed many standards since then.
  2. RFC 2717, and are now defined by RFC 7595, published in June 2015.[28]
  3. ^ For URIs relating to resources on the World Wide Web, some web browsers allow .0 portions of dot-decimal notation to be dropped or raw integer IP addresses to be used.[30]
  4. RFC 1866 (obsoleted by RFC 2854) encourages CGI authors to support ';' in addition to '&'.[32]


  1. ^ Palmer, Sean. "The Early History of HTML". infomesh.net. Retrieved 2020-12-06.
  2. ^ "W3 Naming Schemes". www.w3.org. 1992. Retrieved 2020-12-06.
  3. ^ "Proceedings of the Twenty-Fourth Internet Engineering Task Force" (PDF). p. 193. Retrieved 2021-07-27.
  4. ^ "Proceedings of the Twenty-Fifth Internet Engineering Task Force" (PDF). p. 501. Retrieved 2021-07-27.
  5. doi:10.17487/RFC1630. Retrieved 2020-12-06. {{cite journal}}: Cite journal requires |journal= (help
  6. . Retrieved 2020-12-06.
  7. . Retrieved 2020-12-06.
  8. . Retrieved 2020-12-06.
  9. ^ a b RFC 2396 (1998).
  10. . Retrieved 2020-12-06.
  11. . Retrieved 2020-12-06.
  12. . Retrieved 2020-12-06.
  13. ^ Raman, T.V. (2006-11-01). "On Linking Alternative Representations To Enable Discovery And Publishing". www.w3.org. Retrieved 2020-12-06.
  14. . Retrieved 2020-12-06.
  15. ^ Fielding, Roy (2005-06-18). "[httpRange-14] Resolved". lists.w3.org. Retrieved 2020-12-06.
  16. ^ Sauermann, Leo (December 2008). "Cool URIs for the Semantic Web". www.w3.org. Retrieved 2020-12-06.
  17. ^ URI Planning Interest Group, W3C/IETF (September 2001). "URIs, URLs, and URNs: Clarifications and Recommendations 1.0". www.w3.org. W3C/IETF. Retrieved 2020-12-08.
  18. ^ Joint W3C/IETF URI Planning Interest Group (2002).
  19. ^ "URL Standard: 6.3. URL APIs elsewhere".
  20. ^ "URL Standard: Goals".
  21. ^ RFC 3986 (2005).
  22. ^ RFC 3986 (2005), §2.2.
  23. ^ RFC 3986 (2005), §2.3.
  24. ^ a b RFC 3986 (2005), §2.1.
  25. ^ RFC 3986 (2005), §2.
  26. ^ a b RFC 3986 (2005), §3.
  27. ^ RFC 3986 (2005), §5.2.1.
  28. ^ IETF (2015).
  29. ^ RFC 3986 (2005), §3.2.2.
  30. ^ Lawrence (2014).
  31. ^ RFC 2396 (1998), §3.3.
  32. ^ RFC 1866 (1995), §8.2.1.
  33. ^ RFC 3986 (2005), §A.
  34. ^ RFC 3986 (2005), §4.1.
  35. ^ RFC 3986 (2005), §4.2.
  36. ^ RFC 3986 (2005), §4.4.
  37. ^ RFC 3986 (2005), §5.1.
  38. ^ RFC 3986 (2005), §5.4.
  39. ^ Whitehead 1998, p. 38.
  40. ^ Morrison (2006).
  41. ^ Harold (2004).
  42. ^ W3C (2009).
  43. ^ W3C (2006).

Further reading

External links