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. In July 1992 Berners-Lee's report on theRefinement
In December 1994, 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 IETFhttp
scheme. Simultaneously, the IETF published the content of Design
URLs and URNs
Ahttp://example.org/wiki/Main_Page
refers to a resource identified as /wiki/Main_Page
, whose representation, in the form of example.org
.
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 Syntax
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 , published in August 1998, and finalized in , published in January 2005. A URI is composed from an allowed set of:
, /
, ?
, #
, , ">/code>,
/code>, and @
; scheme- or implementation-specific: !
, $
, &
, '
, (
, )
, *
, +
, ,
, ;
, and =
), ''unreserved characters'' ( uppercase and lowercase letters, decimal digits, -
, .
, _
, and ~
), and the character %
. 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, and percent-encoding
Percent-encoding, also known as URL encoding, is a method to encode arbitrary data in a Uniform Resource Identifier (URI) using only the limited US-ASCII characters legal within a URI. Although it is known as ''URL encoding'', it is also used ...
s 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
Octet may refer to:
Music
* Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble
** String octet, a piece of music written for eight string instruments
*** Octet (Mendelssohn), 1825 compos ...
is a sequence of three characters, consisting of the character %
followed by the two hexadecimal digits representing that octet's numeric value.
The URI generic syntax consists of five ''components'' organized hierarchically in order of decreasing significance from left to right:
URI = scheme ":" //" authoritypath ?" 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. A component is ''empty'' if it has no characters; the scheme component is always non-empty.
The authority component consists of ''subcomponents'':
authority = serinfo "@"host :" port
This is represented in a syntax diagram
Syntax diagrams (or railroad diagrams) are a way to represent a context-free grammar. They represent a graphical alternative to Backus–Naur form, EBNF, Augmented Backus–Naur form, and other text-based grammars as metalanguages. Early books usi ...
as:
The URI comprises:
* A non-empty component followed by a colon (:
), 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 http
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...
, https
Hypertext Transfer Protocol Secure (HTTPS) is an extension of the Hypertext Transfer Protocol (HTTP). It is used for secure communication over a computer network, and is widely used on the Internet. In HTTPS, the communication protocol is enc ...
, ftp
The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and data ...
, 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 directly from an HTML document, without having to copy it and entering i ...
, file
File or filing may refer to:
Mechanical tools and processes
* File (tool), a tool used to ''remove'' fine amounts of material from a workpiece
**Filing (metalworking), a material removal process in manufacturing
** Nail file, a tool used to gent ...
, data
In the pursuit of knowledge, data (; ) is a collection of discrete Value_(semiotics), values that convey information, describing quantity, qualitative property, quality, fact, statistics, other basic units of meaning, or simply sequences of sy ...
and irc
Internet Relay Chat (IRC) is a text-based chat system for instant messaging. IRC is designed for group communication in discussion forums, called '' channels'', but also allows one-on-one communication via private messages as well as chat an ...
. URI schemes should be registered with the Internet Assigned Numbers Authority (IANA), although non-registered schemes are used in practice.
* An optional component preceded by two slashes (//
), comprising:
** An optional subcomponent followed by an at symbol (@
), that may consist of a user name
A user is a person who utilizes a computer or network service.
A user often has a user account and is identified to the system by a username (or user name). Other terms for username include login name, screenname (or screen name), accoun ...
and an optional password
A password, sometimes called a passcode (for example in Apple devices), is secret data, typically a string of characters, usually used to confirm a user's identity. Traditionally, passwords were expected to be memorized, but the large number of ...
preceded by a colon (:
). Use of the format username:password
in 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 subcomponent, consisting of either a registered name (including but not limited to a hostname
In computer networking, a hostname (archaically nodename) is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication, such as the World Wide Web. Hos ...
) or an IP address
An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
. IPv4 addresses must be in dot-decimal notation
Dot-decimal notation is a presentation format for numerical data. It consists of a string of decimal numbers, using the full stop (''dot'') as a separation character.
A common use of dot-decimal notation is in information technology where it is ...
, and IPv6
Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. IPv ...
addresses must be enclosed in brackets ([]
).
** An optional subcomponent preceded by a colon (:
), consisting of decimal digits.
* A component, consisting of a sequence of path segments separated by a slash (/
). 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 defined, then the path component must either be empty or begin with a slash (/
). If an authority component is undefined, then the path cannot begin with an empty segment—that is, with two slashes (//
)—since the following characters would be interpreted as an authority component.
: By convention, in http and https URIs, the last part of a ''path'' is named 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.).
: Example:
:: URI:
:: where: is the first part of the ''path'' (an executable module or program) and is the second part of the ''path'' named ''pathinfo'', which is passed to the executable module or program named 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
Clean URLs, also sometimes referred to as RESTful URLs, user-friendly URLs, pretty URLs or search engine-friendly URLs, are URLs intended to improve the usability and accessibility of a website or web service by being immediately and intuitively ...
' whose last part may be a ' slug'.
* An optional component preceded by a question mark (?
), consisting of a query string
A query string is a part of a uniform resource locator (URL) that assigns values to specified parameters. A query string commonly includes fields added to a base URL by a Web browser or other client application, for example as part of an HTML, cho ...
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.
* An optional component preceded by a hash (#
). 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
The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaSc ...
document, the fragment is often an id
attribute of a specific element, and web browsers will scroll this element into view.
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.
Example URIs
The following figure displays example URIs and their component parts.
DOIs (digital object identifier
A digital object identifier (DOI) is a persistent identifier or handle used to uniquely identify various objects, standardized by the International Organization for Standardization (ISO). DOIs are an implementation of the Handle System; they a ...
s) fit within the Handle System
The Handle System is the Corporation for National Research Initiatives's proprietary registry assigning persistent identifiers, or handles, to information resources, and for resolving "those handles into the information necessary to locate, acces ...
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 (:
). 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
).
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:
* in HTML
The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaSc ...
, 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
Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable ...
, 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.
https://example.com/path/resource.txt#fragment
//example.com/path/resource.txt
/path/resource.txt
path/resource.txt
../resource.txt
./resource.txt
resource.txt
#fragment
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:
* 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
http://a/b/c/d;p?q
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"
URL munging
URL munging is a technique by which a command
Command may refer to:
Computing
* Command (computing), a statement in a computer language
* COMMAND.COM, the default operating system shell and command-line interpreter for DOS
* Command key, a modifier key on Apple Macintosh computer keyboards
* ...
is appended to a URL, usually at the end, after a "?" token. It's commonly used in WebDAV
WebDAV (Web Distributed Authoring and Versioning) is a set of extensions to the Hypertext Transfer Protocol (HTTP), which allows user agents to collaboratively author contents ''directly'' in an HTTP web server by providing facilities for con ...
as a mechanism of adding functionality to HTTP
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...
. 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.
Relation to XML namespaces
In XML
Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable ...
, a namespace
In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified.
Namespaces ...
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
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...
.
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.
See also
* CURIE
In computing, a CURIE (or ''Compact URI'') defines a generic, abbreviated syntax for expressing Uniform Resource Identifiers (URIs). It is an abbreviated URI expressed in a compact syntax, and may be found in both XML and non-XML grammars. A CURIE ...
* Linked data
In computing, linked data (often capitalized as Linked Data) is structured data which is interlinked with other data so it becomes more useful through semantic queries. It builds upon standard Web technologies such as HTTP, RDF and URIs, but ...
* Extensible Resource Identifier
* Internationalized Resource Identifier (IRI)
* Internet resource locator
* Persistent uniform resource locator
A persistent uniform resource locator (PURL) is a uniform resource locator (URL) (i.e., location-based uniform resource identifier or URI) that is used to redirect to the location of the requested web resource. PURLs redirect HTTP clients using ...
* Uniform Naming Convention
A path is a string of characters used to uniquely identify a location in a directory structure. It is composed by following the directory tree hierarchy in which components, separated by a delimiting character, represent each directory. The de ...
* Resource Directory Description Language In computing, Resource Directory Description Language (RDDL) is an extension of XHTML Basic 1.0. An RDDL document, called a Resource Directory, provides a package of information about some target. The targets which RDDL was designed to describe are ...
* Universally unique identifier
A universally unique identifier (UUID) is a 128-bit label used for information in computer systems. The term globally unique identifier (GUID) is also used.
When generated according to the standard methods, UUIDs are, for practical purposes, u ...
* List of URI schemes
This article lists common URI schemes. A Uniform Resource Identifier helps identify a source without ambiguity. Many URI schemes are registered with the IANA; however, there exist many unofficial URI schemes as well. Mobile deep links are one ex ...
Notes
References
Further reading
*
*
*
*
*
*
*
*
*
*
*
*
*
*
External links
URI Schemes
nbsp;– IANA-maintained registry of URI Schemes
URI schemes on the W3C wiki
Architecture of the World Wide Web, Volume One, §2: Identification
nbsp;– by W3C
W3C URI Clarification
{{Use dmy dates, date=May 2020, cs1-dates=y}
Application layer protocols
Internet protocols
Internet Standards
URL