Uniform resource identifier
In computing, a uniform resource identifier (URI) is a string of characters used to identify a name or a web resource. Such identification enables interaction with representations of the web resource over a network (typically the World Wide Web) using specific protocols. Schemes specifying a concrete syntax and associated protocols define each URI.
Relationship to URL and URN 
URIs can be classified as locators (URLs), as names (URNs), or as both. A uniform resource name (URN) functions like a person's name, while a uniform resource locator (URL) resembles that person's street address. In other words: the URN defines an item's identity, while the URL provides a method for finding it.
The ISBN system for uniquely identifying books provides a typical example of the use of URNs. ISBN 0-486-27557-4 (urn:isbn:0-486-27557-4) cites, unambiguously, a specific edition of Shakespeare's play Romeo and Juliet. To gain access to this object and read the book, one needs its location: a URL address. A typical URL for this book on a Unix-like operating system would be a file path such as file:///home/username/books/, identifying the electronic book library saved on a local hard disk. So URNs and URLs have complementary purposes.
Technical view 
A URL is a URI that, in addition to identifying a web resource, specifies the means of acting upon or obtaining the representation: either through description of the primary access mechanism, or through network "location". For example, the URL http://en.wikipedia.org/wiki/Main_Page identifies a resource, in this case English Wikipedia's home page, whose representation, in the form of the home page's current HTML and related code, as encoded characters, is obtainable via the HyperText Transfer Protocol from a network host whose domain name is en.wikipedia.org. A uniform resource name (URN) is a URI that identifies a resource by name, in a particular namespace. One can use a URN to talk about a resource without implying its location or how to access it. For example, the URN urn:isbn:0-395-36341-1 is a URI that specifies the identifier system, i.e. international standard book number (ISBN), as well as the unique reference within that system and allows one to talk about a book, but the URI does not suggest where or how to obtain an actual copy of it.
Technical publications, especially standards produced by the IETF and by the W3C, normally reflect a view outlined in a W3C Recommendation of 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". A URL is simply a URI that happens to point to a physical resource over a network.
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' scheme. Such assumptions can lead to confusion, for example when viewing XML source: the normal means of identifying unique XML vocabularies within an XML document is to declare XML namespaces whose names are URIs that begin with 'http' and use the syntax of a genuine domain name followed by a file path, but which have no need to point to any specific file locations that actually exist.
The URI syntax consists of a URI scheme name (such as "
crid" or "
file") followed by a colon character, and then by a scheme-specific part. The specifications that govern the schemes determine the syntax and semantics of the scheme-specific part. However, URI syntax does require all schemes to adhere to a general syntax that (among other things) reserves certain characters for special purposes (without always identifying those purposes). The URI syntax also enforces restrictions on the scheme-specific part in order to (for example) provide for a degree of consistency when the part has a hierarchical structure.
Percent encoding can add extra information to a URI.
Naming, addressing, and identifying resources 
URIs and URLs have a shared history. In 1994, 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. At the time, people referred to it as a 'hypertext name' or 'document name'.
Over the next three and a half years, as the World Wide Web's core technologies of HTML (the HyperText Markup Language), 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.
During the debate over defining URLs and URNs it became evident that the two concepts embodied by the terms were merely aspects of the fundamental, overarching notion of resource identification. In June 1994, the IETF published Berners-Lee's RFC 1630: the first RFC that (in its non-normative text) acknowledged the existence of URLs and URNs, and, more importantly, defined a formal syntax for Universal Resource Identifiers — URL-like strings whose precise syntaxes and semantics depended on their schemes. In addition, this RFC attempted to summarize the syntaxes of URL schemes in use at the time. It also acknowledged, but did not standardize, the existence of relative URLs and fragment identifiers.
Refinement of specifications 
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. The agreed definition and syntax of URNs had to wait until the publication of RFC 2141 in May 1997.
The publication of RFC 2396 in August 1998 saw the URI syntax become a separate specification and most of the parts of RFCs 1630 and 1738 relating to URIs and URLs in general were revised and expanded by the IETF. The new RFC changed the significance of the "U" in "URI": it came to represent "Uniform" rather than "Universal". The sections of RFC 1738 that summarized existing URL schemes migrated into a separate document. IANA keeps a registry of those schemes; RFC 2717 first described the procedure to register them.
In December 1999, RFC 2732 provided a minor update to RFC 2396, allowing URIs to accommodate IPv6 addresses. Some time later, a number of shortcomings discovered in the two specifications led to the development of a number of draft revisions under the title rfc2396bis. This community effort, coordinated by RFC 2396 co-author Roy Fielding, culminated in the publication of RFC 3986 in January 2005. This RFC, as of 2009[update] the current version of the URI syntax recommended for use on the Internet, renders RFC 2396 obsolete. It does not, however, render the details of existing URL schemes obsolete; RFC 1738 continues to govern such schemes except where otherwise superseded – RFC 2616 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 August 2002, RFC 3305 pointed out that the term 'URL' has, despite its widespread use in the vernacular of the Internet-aware public at large, faded into near obsolescence. It now serves only as a reminder that some URIs act as addresses because they have schemes that imply some kind of network accessibility, regardless of whether systems actually use them for that purpose. 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.
On November 1, 2006, the W3C Technical Architecture Group published 'On Linking Alternative Representations To Enable Discovery And Publishing', a guide to best practices and canonical URIs for publishing multiple versions of a given resource. For example, content might differ by language or by size to adjust for capacity or settings of the device used to access that content.
The Semantic Web uses the HTTP URI scheme to identify both documents and concepts in the real world: this has caused confusion as to how to distinguish the two. The Technical Architecture Group of W3C (TAG) published an e-mail in June 2005 on how to solve this problem. The e-mail became known as the httpRange-14 resolution. To expand on this (rather brief) email, W3C published in March 2008 the Interest Group Note Cool URIs for the Semantic Web. This explains the use of content negotiation and the 303-redirect code in more detail.
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
srcattribute of the
imgelement provides a URI reference, as does the value of the
hrefattribute of the
- In XML, the system identifier appearing after the
SYSTEMkeyword in a DTD is a fragmentless URI reference.
- In XSLT, the value of the
hrefattribute of the
xsl:importelement/instruction is a URI reference; likewise the first argument to the
Examples of absolute URIs 
Examples of URI references 
- http://en.wikipedia.org/wiki/URI#Examples_of_URI_references ("http" specifies the 'scheme' name, "en.wikipedia.org" is the 'authority', "/wiki/URI" the 'path' pointing to this article, and "#Examples_of_URI_references" is a 'fragment' pointing to this section.)
- (empty string)
URI resolution 
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.
One can regard a URI reference as a same document reference: a reference to the document containing the URI reference itself. Document processing software can efficiently use its current representation of the document to satisfy the resolution of a same document reference without fetching a new representation. This is only a recommendation, and document processing software can alternatively use other mechanisms to determine whether to obtain a new representation.
The current URI specification as of 2009[update], RFC 3986, defines a URI reference as a same document reference if, when resolved to absolute form, it equates exactly to the base URI in effect for the reference. Typically, the base URI is the URI of the document containing the reference. XSLT 1.0, for example, has a
document() function that, in effect, implements this functionality. RFC 3986 also formally defines URI equivalence, which can be used to determine that a URI reference, while not identical to the base URI, still represents the same resource and thus can be considered to be a same document reference.
RFC 2396 prescribed a different method for determining same document references; RFC 3986 made RFC 2396 obsolete, but RFC 2396 still serves as the basis of many specifications and implementations. This specification defines a URI reference as a same document reference if it is an empty string or consists of only the # character followed by an optional fragment.
Relation to XML namespaces 
XML has a concept of a namespace, an abstract domain to which a collection of element and attribute names can be assigned. The namespace name (a character string which must adhere to the generic URI syntax) identifies an XML namespace. However, the namespace name is generally not considered to be a URI because the 'URI-ness' of strings is, according to the URI specification, based on their intended use, not just their lexical components. A namespace name also does not necessarily imply any of the semantics of URI schemes; a namespace name beginning with 'http:', for example, likely has nothing to do with the HTTP protocol. XML professionals have debated this thoroughly on the xml dev electronic mailing list; some feel that a namespace name could be a URI, since the collection of names comprising a particular namespace could be regarded as a resource that is being identified, and since a version of the 'Namespaces in XML' specification says that the namespace name is a URI reference. But the consensus seems to suggest that a namespace name is just a string that happens to look like a URI, nothing more.
Initially, the namespace name could match the syntax of any non-empty URI reference, but an erratum to the 'Namespaces In XML Recommendation' later deprecated the use of relative URI references. A separate specification, issued for namespaces for XML 1.1, allows IRI references, not just URI references, to serve as the basis for namespace names.
To mitigate confusion that began to arise among newcomers to XML from the use of URIs (particularly HTTP URLs) for namespaces, a descriptive language called RDDL (Resource Directory Description Language) developed, though the specification of RDDL has no official standing and no relevant organization (such as W3C) has considered or approved it. An RDDL document can provide machine- and human-readable information about a particular namespace and about the XML documents that use it. Authors of XML documents were encouraged[by whom?] to put RDDL documents in locations such that if a namespace name in their document somehow becomes de-referenced, then an RDDL document would be obtained, thus satisfying the desire among many developers for a namespace name to point to a network-accessible resource.
See also 
- URI Planning Interest Group, W3C/IETF (21 September 2001). "URIs, URLs, and URNs: Clarifications and Recommendations 1.0". Retrieved 2009-07-27.
- Much of this discussion comes from RFC3305, titled 'Report from the Joint W3C/IETF URI Planning Interest Group: Uniform Resource Identifiers (URIs), URLs, and Uniform Resource Names (URNs): Clarifications and Recommendations'. This RFC outlines the work of a joint W3C/IETF working group set up specifically 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.
- Morrison, Michael (2006). "Hour 5: Putting Namespaces to Use". Sams Teach Yourself XML. Sams Publishing. p. 91.
- Palmer, Sean B. "The Early History of HTML". Retrieved 2009-04-30.
- "W3 Naming Schemes". W3. Retrieved 2009-07-24. "The format of a hypertext name consists of the name of the naming sub-scheme to be used, then a name in a format particular to that sub-scheme, then an optional anchor identifier within the document. For example, the format is for all internet-based access methods:
scheme : // host.domain:port / path / path # anchor"
- "FAQS.org". FAQS.org. Retrieved 2012-04-03.
- This separate document is not explicitly linked[by whom?], RFC 2717 and RFC 4395 point to the IANA registry as the official URI scheme registry.
- "IANA registry of URI schemes". Iana.org. 2012-03-26. Retrieved 2012-04-03.
- "w3.org". w3.org. Retrieved 2012-04-03.
- The httpRange-14 resolution consists of three bullet points: see Fielding, Roy T. (2005-06-18). "[httpRange-14] Resolved". Retrieved 2009-07-24., and did not help much to reduce the confusion.
- "W3.org". W3.org. Retrieved 2012-04-03.
- Harold, Elliote Rusty (2004). XML 1.1 Bible, Third Edition, Wiley Publishing Inc., p. 291. ISBN 0-7645-4986-3.
- World Wide Web Consortium (1999-01-14). "Namespaces in XML" (PDF). W3C. Retrieved 2009-09-14. "[Definition:] The attribute's value, a URI reference, is the namespace name identifying the namespace."
- "rddl.org". rddl.org. Retrieved 2012-04-03.
||This article's use of external links may not follow Wikipedia's policies or guidelines. (April 2013)|
- RFC 3986 / STD 66 (2005) – the current[update] generic URI syntax specification
- URI Schemes – IANA-maintained registry of URI Schemes
- Architecture of the World Wide Web, Volume One, §2: Identification – by W3C
- W3C URI Clarification