Uniform Resource Identifier

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 Intranet), these are Uniform Resource Locators (URLs). Other URIs provide only a unique name, without a means of locating or retrieving the resource or information about it, these are Uniform Resource Names (URNs). The web technologies that use URIs are not limited to web browsers. URIs are used to identify anything described using the Resource Description Framework (RDF), for example, concepts that are part of an ontology defined using the Web Ontology Language (OWL), and people who are described using the Friend of a Friend vocabulary would each have an individual URI.
History.
Conception.
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. 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, 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 concepts embodied by the two terms were merely aspects of the fundamental, overarching, notion of resource "identification". In June 1994, the IETF published Berners-Lee's first "Request for Comments" that acknowledged the existence of URLs and URNs. Most importantly, it defined a formal syntax for "Universal Resource Identifiers" (i.e. URL-like strings whose precise syntaxes and semantics depended on their schemes). In addition, the RFC (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.
Refinement.
In December 1994, IETF 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 IETF RFC 2141 in May 1997.
The publication of IETF 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 meaning of "U" in "URI" to "Uniform" from "Universal".
In December 1999, IETF RFC 2732 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 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 for example, refines the codice_1 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. 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 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". 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.
Design.
URLs and URNs.
A Uniform Resource Name (URN) is a URI that identifies a resource by name in a particular namespace. A URN may be used to talk about a resource without implying its location or how to access it. For example, in the 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.
A Uniform Resource Locator (URL) is a URI that specifies the means of acting upon or obtaining the representation of a resource, i.e. specifying both its primary access mechanism and network location. For example, the URL codice_2 refers to a resource identified as codice_3, whose representation, in the form of HTML and related code, is obtainable via the Hypertext Transfer Protocol ("http:") from a network host whose domain name is codice_4.
A URN may be compared to a person's name, while a URL may be compared 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 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. 
As such, a URL is simply a URI that happens to point to a 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" 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". 
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.
Syntax.
Each URI begins with a scheme name 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 , published in August 1998, and finalized in , published in January 2005.
<section begin="syntax"/>
The "URI generic syntax" consists of a hierarchical sequence of five "components":
where the authority component divides into three "subcomponents":
This is represented in a syntax diagram as:
The URI comprises:
Strings of data octets within a URI are represented as characters. Permitted characters within a URI are the ASCII characters for the lowercase and uppercase letters of the modern English alphabet, the Arabic numerals, hyphen, period, underscore, and tilde. Octets represented by any other character must be percent-encoded.
Of the ASCII character set, the characters codice_30 are reserved for use as delimiters of the generic URI components and must be percent-encoded – for example, codice_31 for a question mark. The characters codice_32 are permitted by generic URI syntax to be used unencoded in the user information, host, and path as delimiters. Additionally, codice_5 and codice_18 may appear unencoded within the path, query, and fragment; and codice_27 and codice_23 may appear unencoded as data within the query or fragment.
The following figure displays example URIs and their component parts.
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 (codice_5). A path segment that contains a colon character (e.g., codice_38) cannot be used as the first path segment of a relative reference if its path component does not begin with a slash (codice_23), as it would be mistaken for a scheme component. Such a path segment must be preceded by a dot path segment (e.g., codice_40).
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:
Resolution.
"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:
Within a representation with a well defined base URI of
a relative reference is resolved to its target URI as follows:
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. However, the name is generally not considered to be a URI, 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. 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.