A Uniform Resource Identifier (URI) is a string of characters that unambiguously identifies a particular resource. To guarantee uniformity, all URIs follow a predefined set of syntax rules, but also maintain extensibility through a separately defined hierarchical naming scheme (e.g.
Such identification enables interaction with representations of the resource over a network, typically the World Wide Web, using specific protocols. Schemes specifying a concrete syntax and associated protocols define each URI. The most common form of URI is the Uniform Resource Locator (URL), frequently referred to informally as a web address. More rarely seen in usage is the Uniform Resource Name (URN), which was designed to complement URLs by providing a mechanism for the identification of resources in particular namespaces.
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
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, is obtainable via the Hypertext Transfer Protocol (http:) from a network host whose domain name is
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.[a] 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.
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.
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 Request for Comments (RFC) 2396, published in August 1998, and finalized in RFC 3986, published in January 2005.
The URI generic syntax consists of a hierarchical sequence of five components:
URI = scheme:[//authority]path[?query][#fragment]
where the authority component divides into three subcomponents:
authority = [userinfo@]host[:port]
This is represented in a syntax diagram as:
The URI comprises:
:), consisting of a sequence of characters beginning with a letter and followed by any combination of letters, digits, plus (
+), period (
.), or hyphen (
-). Although schemes are case-insensitive, the canonical form is lowercase and documents that specify schemes must do so with lowercase letters. Examples of popular schemes include
irc. URI schemes should be registered with the Internet Assigned Numbers Authority (IANA), although non-registered schemes are used in practice.[b]
:), followed by an at symbol (
@). Use of the format
username:passwordin the userinfo subcomponent is deprecated for security reasons. Applications should not render as clear text any data after the first colon (
:) found within a userinfo subcomponent unless the data after the colon is the empty string (indicating no password).
/). A path is always defined for a URI, though the defined path may be empty (zero length). A segment may also be empty, resulting in two consecutive slashes (
//) in the path component. A path component may resemble or map exactly to a file system path, but does not always imply a relation to one. If an authority component is present, then the path component must either be empty or begin with a slash (
/). If an authority component is absent, then the path cannot begin with an empty segment, that is with two slashes (
//), as the following characters would be interpreted as an authority component. The final segment of the path may be referred to as a 'slug'.
?), containing a query string of non-hierarchical data. Its syntax is not well defined, but by convention is most often a sequence of attribute–value pairs separated by a delimiter.
#). The fragment contains a fragment identifier providing direction to a secondary resource, such as a section heading in an article identified by the remainder of the URI. When the primary resource is an HTML document, the fragment is often an
idattribute of a specific element, and web browsers will scroll this element into view.
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
: / ? # [ ] @ are reserved for use as delimiters of the generic URI components and must be percent-encoded – for example,
%3F for a question mark. The characters
! $ & ' ( ) * + , ; = are permitted by generic URI syntax to be used unencoded in the user information, host, and path as delimiters. Additionally,
@ may appear unencoded within the path, query, and fragment; and
/ may appear unencoded as data within the query or fragment.
The following figure displays example URIs and their component parts.
userinfo host port ┌─┴────┐ ┌────┴────────┐ ┌┴┐ https://email@example.com:123/forum/questions/?tag=networking&order=newest#top └─┬─┘ └───────┬────────────────────┘└─┬─────────────┘└──┬───────────────────────┘└┬─┘ scheme authority path query fragment ldap://[2001:db8::7]/c=GB?objectClass?one └─┬┘ └───────┬─────┘└─┬─┘ └──────┬──────┘ scheme authority path query mailto:John.Doe@example.com └──┬─┘ └─────────┬────────┘ scheme path news:comp.infosystems.www.servers.unix └─┬┘ └───────────────┬───────────────┘ scheme path tel:+1-816-555-1212 └┬┘ └──────┬──────┘ scheme path telnet://192.0.2.16:80/ └──┬─┘ └──────┬──────┘│ scheme authority path urn:oasis:names:specification:docbook:dtd:xml:4.1.2 └┬┘ └──────────────────────┬──────────────────────┘ scheme path
A URI reference is either a URI, or a relative reference when it does not begin with a scheme component followed by a colon (
:). 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.,
srcattribute of the
imgelement provides a URI reference, as does the value of the
hrefattribute of the
SYSTEMkeyword in a DTD is a fragmentless URI reference;
hrefattribute of the
xsl:importelement/instruction is a URI reference; likewise the first argument to the
https://example.com/path/resource.txt#fragment //example.com/path/resource.txt /path/resource.txt path/resource.txt /path/resource.txt ../resource.txt ./resource.txt resource.txt #fragment
An absolute URI is a URI with no fragment component.
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. 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:
"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"
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, 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 Request for Comments that 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.
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 meaning of "U" in "URI" to "Uniform" from "Universal".
In December 1999, 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 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. 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 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, 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.
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.
.0portions of dot-decimal notation to be dropped or raw integer IP addresses to be used.
The data URI scheme is a uniform resource identifier (URI) scheme that provides a way to include data in-line in Web pages as if they were external resources. It is a form of file literal or here document. This technique allows normally separate elements such as images and style sheets to be fetched in a single Hypertext Transfer Protocol (HTTP) request, which may be more efficient than multiple HTTP requests, and used by several browser extensions to package images as well as other multimedia contents in a single HTML file for page saving. As of 2015, data URIs are fully supported by most major browsers, and partially supported in Internet Explorer and Microsoft Edge.Feed URI scheme
The feed URI scheme was a suggested uniform resource identifier (URI) scheme designed to facilitate subscription to web feeds; specifically, it was intended that a news aggregator be launched whenever a hyperlink to a feed URI was clicked in a web browser.
The scheme was intended to flag a document in a syndication format such as Atom or RSS. The document would be typically served over HTTP.by open wave and sms http based html Uri as feed.Geo URI scheme
The geo URI scheme is a Uniform Resource Identifier (URI) scheme defined by the Internet Engineering Task Force's RFC 5870 (published 8 June 2010) as:
a Uniform Resource Identifier (URI) for geographic locations using the 'geo' scheme name. A 'geo' URI identifies a physical location in a two- or three-dimensional coordinate reference system in a compact, simple, human-readable, and protocol-independent way.
The current revision of the vCard specification supports geo URIs in a vCard's "GEO" property, and the GeoSMS standard uses geo URIs for geotagging SMS messages. Android based devices support geo URIs, although that implementation is based on a draft revision of the specification, and supports a different set of URI parameters and query strings.
A geo URI is not to be confused with the site GeoUrl (which implements ICBM address).IPKall
IPKall was a public switched telephone network to voice over IP call forwarding service. Users were able to register with the service to obtain a phone number chosen from several Washington State area codes and have all calls to that number forwarded to their Session Initiation Protocol or Inter-Asterisk eXchange uniform resource identifier, including an Asterisk server.
The telephone company that provided IPKall numbers is International Telcom; they also provide a flat-rate $1/month area code 206 inbound fax service as Faxaway, a non-free voice and fax service as Kall8 (with toll-free or US local numbers) and the Kallback and KallCents discount outbound long-distance services.
IPKall's popularity stemmed from the fact that it was probably the last remaining, widely accessible US-based VoIP service offering free-of-charge PSTN numbers.Info URI scheme
In computing, info is a Uniform Resource Identifier (URI) scheme which enables identifiers from public namespaces to be represented as URIs, when they would otherwise have no canonical URL form, such as Library of Congress identifiers, Handle System handles, and Digital object identifiers.Internationalized Resource Identifier
The Internationalized Resource Identifier (IRI) – is an internet protocol standard which extends the ASCII characters subset of the Uniform Resource Identifier (URI) protocol. It was defined by the Internet Engineering Task Force (IETF) in 2005 as a new internet standard to extend the existing URI scheme. The primary standard is defined by the RFC 3987. While URIs are limited to a subset of the ASCII character set, IRIs may contain characters from the Universal Character Set (Unicode/ISO 10646), including Chinese or Japanese kanji, Korean, Cyrillic characters, and so forth.Linked data
In computing, linked data (often capitalized as Linked Data) is a structured data which is interlinked with other data so that become more useful through semantic queries. It builds upon standard Web technologies such as HTTP, RDF and URIs, but rather than using them to serve web pages only for human readers, it extends them to share information in a way that can be read automatically by computers. Part of the vision of linked data is for the internet to become a global database.
Tim Berners-Lee, director of the World Wide Web Consortium (W3C), coined the term in a 2006 design note about the Semantic Web project.Linked data may also be open data, in which case it is usually described as linked open data (LOD).Mailto
mailto is a Uniform Resource Identifier (URI) scheme for email addresses. It is used to produce hyperlinks on websites that allow users to send an email to a specific address without first having to copy it and enter it into an email client. It was originally defined in Request for Comments (RFC) 2368, published in July 1998, and refined in RFC 6068, published in October 2010.Microsoft Media Server
Microsoft Media Server (MMS), a Microsoft proprietary network-streaming protocol, serves to transfer unicast data in Windows Media Services (previously called NetShow Services). MMS can be transported via UDP or TCP. The MMS default port is UDP/TCP 1755.Microsoft deprecated MMS in favor of RTSP (TCP/UDP port 554) in 2003 with the release of the Windows Media Services 9 Series, but continued to support the MMS for some time in the interest of backward compatibility. Support for the protocol was finally dropped in Windows Media Services 2008.As of 2012 Microsoft still recommends using "mms://" as a "protocol rollover URL". As part of protocol rollover a Windows Media Player version 9, 10, or 11 client opening an "mms://" URL will attempt to connect first with RTSP over UDP and if that fails it will attempt RTSP over TCP. After an RTSP attempt fails, Windows Media Player versions 9 and 10 will attempt MMS over UDP, then MMS over TCP. If using Windows Media Player 11 and an RTSP attempt fails, or if using a previous version of Windows Media Player and MMS fails, a modified version of a HTTP over TCP connection will be attempted. This modified version is referred to by some third parties as MMSH, and by Microsoft as MS-WMSP (Windows Media HTTP Streaming Protocol). The uniform resource identifier (URI) scheme mms has also been proposed to be used for the unrelated Multimedia Messaging Service (MMS) protocol.For several years developers of the SDP Multimedia download-tool reverse engineered the MMS protocol and published unofficial documentation for it. However, Microsoft finally released the protocol specification in February 2008.Mobile deep linking
In the context of mobile apps, deep linking consists of using a uniform resource identifier (URI) that links to a specific location within a mobile app rather than simply launching the app. Deferred deep linking allows users to deep link to content even if the app is not already installed. Depending on the mobile device platform, the URI required to trigger the app may be different. See List of Supported Deep Links.Percent-encoding
Percent-encoding, also known as URL encoding, is a mechanism for encoding information in a Uniform Resource Identifier (URI) under certain circumstances. Although it is known as URL encoding it is, in fact, used more generally within the main Uniform Resource Identifier (URI) set, which includes both Uniform Resource Locator (URL) and Uniform Resource Name (URN). As such, it is also used in the preparation of data of the application/x-www-form-urlencoded media type, as is often used in the submission of HTML form data in HTTP requests.QRpedia
QRpedia is a mobile Web-based system which uses QR codes to deliver Wikipedia articles to users, in their preferred language. QR codes can easily be generated to link directly to any Uniform Resource Identifier (URI), but the QRpedia system adds further functionality. It is owned and operated by a subsidiary of Wikimedia UK (WMUK).
QRpedia was conceived by Roger Bamkin, a Wikipedia volunteer, coded by Terence Eden, and unveiled in April 2011. It is currently in use at museums and other institutions in countries including Australia, Bulgaria, the Czech Republic, Estonia, Macedonia, Spain, India, the United Kingdom, Germany, Ukraine and the United States. The project's source code is freely reusable under the MIT License.SIP URI scheme
The SIP URI scheme is a Uniform Resource Identifier (URI) scheme for the Session Initiation Protocol (SIP) multimedia communications protocol. A SIP address is a URI that addresses a specific telephone extension on a voice over IP system. Such a number could be a private branch exchange or an E.164 telephone number dialled through a specific gateway. The scheme was defined in RFC 3261.SURBL
SURBL (previously stood for Spam URI RBL) is a collection of URI DNSBL lists of Uniform Resource Identifier (URI) hosts, typically web site domains, that appear in unsolicited messages. SURBL can be used to search incoming e-mail message bodies for spam payload links to help evaluate whether the messages are unsolicited. For example, if http://www.example.com is listed, then e-mail messages with a message body containing this URI may be classified as unsolicited. URI DNSBLs differ from prior DNSBLs, which commonly list mail sending IP addresses. SURBL is a specific instance of the general URI DNSBL list type.Tag URI scheme
The tag URI scheme is a uniform resource identifier (URI) scheme for unique identifiers called tags, defined by RFC 4151 in October 2005.The RFC identifies four requirements for tags:
Identifiers are likely to be unique across space and time, and come from a practically inexhaustible supply.
Identifiers are relatively convenient for humans to mint (create), read, type, remember etc.
No central registration is necessary, at least for holders of domain names or email addresses; and there is negligible cost to mint each new identifier.
The identifiers are independent of any particular resolution scheme.Tags are used extensively in YAML.Tempuri
tempuri.org is the test default namespace URI used by Microsoft development products, like Visual Studio. It is available for XML Web services that are under development, but published XML Web services should use a more permanent namespace. The term is strictly a placeholder and all instances of it should be replaced with a more meaningful URI in production systems.
The World Wide Web Consortium recommends that XML namespaces be a Uniform Resource Identifier. "tempuri" is short for Temporary Uniform Resource Identifier.
An XML Web service should be identified by a namespace that is controlled by its company. For example, a company's Internet domain name could be used as part of the namespace. Although many XML Web service namespaces look like URLs, they need not point to actual resources on the Web. (XML Web service namespaces are URIs.)
For XML Web services created using ASP.NET, the default namespace can be changed using the WebService attribute's Namespace property. The WebService attribute is an attribute applied to the class that contains the XML Web service methods.
The tempuri.org domain is owned by Microsoft and redirects to their Bing search engine.
Tempuri.com also redirects to a search for Tempuri on Bing.URI record
In the Domain Name System, a Uniform Resource Identifier (URI) record (RFC 7553) is a means for publishing mappings from hostnames to URIs.URL
A Uniform Resource Locator (URL), colloquially termed a web address, is a reference to a web resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identifier (URI), although many people use the two terms interchangeably. URLs occur most commonly to reference web pages (http), but are also used for file transfer (ftp), email (mailto), database access (JDBC), and many other applications.
Most web browsers display the URL of a web page above the page in an address bar. A typical URL could have the form http://www.example.com/index.html, which indicates a protocol (http), a hostname (www.example.com), and a file name (index.html).Uniform Resource Name
A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) that uses the urn scheme.
Uniform Resource Identifier (URI) schemes