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|Domain||World 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
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 the
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
In December 1994,
The publication of IETF RFC 2396
In December 1999,
httpscheme. 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
In August 2002, IETF
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.
URLs and URNs
example.org. (In this case, HTTP usually implies it to be in the form of HTML
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
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.
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.
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.
While most URI schemes were originally designed to be used with a particular
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
A URI is composed from an allowed set of ASCII characters consisting of reserved characters (generic:
@; scheme- or implementation-specific:
=), unreserved characters (uppercase and lowercase letters, decimal digits,
~), 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-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.
The URI generic syntax consists of five components organized hierarchically in order of decreasing significance from left to right:
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. A component is empty if it has no characters; the scheme component is always non-empty.
The authority component consists of subcomponents:
authority = [userinfo "@"] host [":" port]
This is represented in a syntax diagram as:
The URI comprises:
- A non-empty scheme 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
- An optional authority component preceded by two slashes (
- An optional userinfo subcomponent followed by an at symbol (
@), that may consist of a user name and an optional password preceded by a colon (
:). 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 host subcomponent, consisting of either a registered name (including but not limited to a
- An optional port subcomponent preceded by a colon (
:), consisting of decimal digits.
- An optional userinfo subcomponent followed by an at symbol (
- A path 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 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.).
"/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'.
- An optional query component preceded by a question mark (
?), consisting of aattribute–value pairs separated by a delimiter.
- An optional fragment component preceded by a
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
= 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
/ may be used in the path, query and fragment, and the generic reserved character
? may be used in the query and fragment.
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 ldap://[2001:db8::7]/c=GB?objectClass?one └┬─┘ └─────┬─────┘└─┬─┘ └──────┬──────┘ scheme authority path query mailto:[email protected] └─┬──┘ └────┬─────────────┘ 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.,
- in HTML, the value of the
srcattribute of the
imgelement provides a URI reference, as does the value of the
hrefattribute of the
- in DTDis 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
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
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
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 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.
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.
- Linked data
- Extensible Resource Identifier
- Internationalized Resource Identifier (IRI)
- Internet resource locator
- Persistent uniform resource locator
- Uniform Naming Convention
- Resource Directory Description Language
- Universally unique identifier
- List of URI schemes
- 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.
- For URIs relating to resources on the World Wide Web, some web browsers allow
.0portions of dot-decimal notation to be dropped or raw integer IP addresses to be used.
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- RFC 3986 (2005), §2.2.
- RFC 3986 (2005), §2.3.
- RFC 3986 (2005), §2.1.
- RFC 3986 (2005), §2.
- RFC 3986 (2005), §3.
- RFC 3986 (2005), §5.2.1.
- IETF (2015).
- RFC 3986 (2005), §3.2.2.
- Lawrence (2014).
- RFC 2396 (1998), §3.3.
- RFC 1866 (1995), §8.2.1.
- RFC 3986 (2005), §A.
- RFC 3986 (2005), §4.1.
- RFC 3986 (2005), §4.2.
- RFC 3986 (2005), §4.4.
- RFC 3986 (2005), §5.1.
- RFC 3986 (2005), §5.4.
- Whitehead 1998, p. 38.
- Morrison (2006).
- Harold (2004).
- W3C (2009).
- W3C (2006).
- ISBN 978-0-7645-4986-1.
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- Morrison, Michael Wayne (2006). "Hour 5: Putting Namespaces to Use". Sams Teach Yourself XML. Sams Publishing. p. 91.
- URI Planning Interest Group, W3C/IETF (2001-09-21). "URIs, URLs, and URNs: Clarifications and Recommendations 1.0". Retrieved 2009-07-27.
- "On Linking Alternative Representations To Enable Discovery And Publishing". World Wide Web Consortium. 2006 . Retrieved 2012-04-03.
- Bray, Tim; Hollander, Dave; Layman, Andrew; Tobin, Richard, eds. (2006-08-16). "Namespaces in XML 1.1 (Second Edition)". World Wide Web Consortium. 2.2 Use of URIs as Namespace Names. Retrieved 2015-08-31.
- Ayers, Danny; Völkel, Max (2008-12-03). Sauermann, Leo; Cyganiak, Richard (eds.). "Cool URIs for the Semantic Web". World Wide Web Consortium. Retrieved 2012-04-03.
- Bray, Tim; Hollander, Dave; Layman, Andrew; Tobin, Richard; Thompson, Henry S., eds. (2009-12-08). "Namespaces in XML 1.0 (Third Edition)". World Wide Web Consortium. 2.2 Use of URIs as Namespace Names. Retrieved 2015-08-31.
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- URI Schemes – IANA-maintained registry of URI Schemes
- URI schemes on the W3C wiki
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- W3C URI Clarification