Uniform Resource Locator

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URL redirects here. For other uses of URL, see URL (disambiguation).

A Uniform Resource Locator, URL (properly pronounced as a spelled-out initialism, not syllabized as 'earl'), or Web address, is a standardized address name layout for resources (such as documents or images) on the Internet (or elsewhere). First created by Tim Berners-Lee for use on the World Wide Web, the currently used forms are detailed by Internet standard RFC 1738. It is also known as Universal Resource Locator [1],[2],[3].

The URL was a fundamental innovation in the history of the Internet. The syntax is designed to be generic, extensible, and able to express addresses in any character set using a limited subset of ASCII characters (for instance, whitespace is never used in a URL). URLs are classified by the "scheme" which typically identifies the network protocol used to retrieve the resource over a computer network.

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Definition

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URIs and URLs

Every URL is a type of Uniform Resource Identifier (URI), or more precisely the set of URLs is a proper subset of the set of URIs. A URI identifies a particular resource while a URL both identifies a resource and indicates how to locate it. To illustrate the distinction consider the URI urn:ietf:rfc:1738 which identifies IETF RFC 1738 without indicating where to find the text of this RFC. Now consider three URLs for three separate documents containing the text of this RFC:

Each URL uniquely identifies each document and thus is a URI itself, but URL syntax is such that the identity allows one to also locate each of these documents. Thus, a URL functions as the document's address.

Historically, the terms have been almost synonymous as almost all URIs have also been URLs. For this reason, many definitions in this article mention URIs instead of URLs; the discussion applies to both URIs and URLs.

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URL scheme

A URL begins with the name of its scheme, followed by a colon, followed by a scheme-specific part.

Some examples of URL schemes:

See also http://www.iana.org/assignments/uri-schemes

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Generic URI syntax

The syntax of the scheme-specific part depends on the requirements of the scheme. Schemes using typical connection-based protocols use a common "generic URI" syntax, defined below:

scheme://authority/path?query

The authority typically consists of a hostname or IP address of a server, optionally followed by a colon and a port number. It may in fact also contain information on username and password for authenticating to the server.

The path is a specification of a location in some hierarchical structure, using a slash ("/") as delimiter between components.

The query part is typically intended to express parameters of a dynamic query to some database residing on the server.

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Example: HTTP URLs

The URLs employed by HTTP, the protocol used to transmit web pages, are the most popular kind of URI and can be used as an example to demonstrate the concept of the URI. The HTTP URL syntax is:

scheme://username:password@host:port/path?parameter=value#anchor
  • scheme, in the case of HTTP, is most of the time http, but https can also be used for signifying HTTP over a TLS connection (to make the connection more secure).
  • The parts username: and password@ can be used for HTTP authentication. If no authentication is desired (which is usually the case), they can be omitted.

==> Note: According to section 3.3 of RFC 1738, "No user name or password is allowed" in HTTP URLs.

  • host, which is probably the most prominent part of a URL, is in almost all cases the domain name of a server, e.g. www.wikipedia.org, google.com, www.imv.au.dk, etc.
  • The :port portion specifies an IP port number. It is usually omitted (defaults to 80 in that case) and in the entire URL probably has the least relevance for the user.
  • The path portion is used by the server (specified by host) in whatever way the server's software is set up, but in many cases it specifies a filename, possibly prepended with directory names. For example, in the path /wiki/Cow, wiki would be a (pseudo-)directory and Cow would be a (pseudo-)filename.
  • The part given above as ?parameter=value is referred to as query portion (sometimes search portion). It can either be omitted, have one parameter-value pair as in the example, or have many of them, which is expressed as ?para=value&anotherpara=value&.... The parameter-value pairs are only relevant if the file specified by the path is not a simple, static webpage, but some sort of automatically generated page. The generator software uses the parameter-value pairs in any way it is set up; mostly they carry information specific to one user and one moment in the use of the site, like concrete search terms, usernames, etc. (Watch, for example, how the URL in your browser's address bar behaves during a Google search: your search term is passed to some sophisticated program on google.com as a parameter, and Google's program returns a page with the search results to you.)
  • The #anchor part, lastly, is called fragment identifier and refers to certain significant places inside a page; for example, this Wikipedia page has anchors at each section heading which can be referred to via the fragment ID. They are relevant if a URL should be given which, when loaded in a browser, directly jumps to a certain point in a long page. An example would be this link, which leads to this page and to the beginning of this section. (Watch how the URL in your browser's address bar changes when clicking the link.)

For another example of a HTTP URL, see below.

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URI references

The term URI reference means a particular instance of a URI, or portion thereof, as used in, for instance, an HTML document, in order to refer to a particular resource. A URI reference often looks just like a URL or the tail end of a URL. URI references introduce two new concepts: the distinction between absolute and relative references, and the concept of a fragment identifier.

An absolute URL is a URI reference that is just like a URL defined above; it starts with a scheme followed by a colon and then a scheme-specific part. A relative URL is a URI reference that comprises just the scheme-specific part of a URL, or some trailing component thereof. The scheme and leading components are inferred from the context in which the URL reference appears: the base URI (or base URL) of the document containing the reference.

A URI reference can also be followed by a hash sign ("#") and a pointer to within the resource referenced by the URI as a whole. This is not a part of the URI as such, but is intended for the "user agent" (browser) to interpret after a representation of the resource has been retrieved. Therefore, it is not supposed to be sent to the server in HTTP requests.

Examples of absolute URLs:

  • http://en.wikipedia.org/w/wiki.phtml?title=Train&action=history
  • http://en.wikipedia.org/wiki/Train#Freight_trains

Examples of relative URLS:

  • //nl.wikipedia.org/wiki/Train
  • /wiki/Train
  • Train#Passenger_trains
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Case-sensitivity

URLs in general are case-sensitive; however it is up to the server administrator to decide to respect case when responding to requests. For convenience some webservers send the same page for URLs differing only in case.

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URLs in everyday use

A HTTP URL combines into one simple address the four basic items of information necessary to retrieve a resource from anywhere on the Internet:

  • the protocol to use to communicate,
  • the host (server) to communicate with,
  • the network port on the server to connect to,
  • the path to the resource on the server (for example, its file name).

A typical URL can look like:

http://en.wikipedia.org:80/wiki/Special:Search?search=train&go=Go

where

  • http is the protocol,
  • en.wikipedia.org is the host,
  • 80 is the network port number on the server (as 80 is the default value for the HTTP protocol, this portion could have been omitted entirely),
  • /wiki/Special:Search is the resource path,
  • ?search=train&go=Go is the query string; this part is optional.

Most web browsers do not require the user to enter "http://" to address a webpage, as HTTP is by far the most common protocol used in web browsers. Likewise, since 80 is the default port for http it is not usually specified. One usually just enters a partial URL such as www.wikipedia.org/wiki/Train. To go to a homepage one usually just enters the host name, such as www.wikipedia.org.

Since the HTTP protocol allows a server to respond to a request by redirecting the web browser to a different URL, many servers additionally allow users to omit certain parts of the URL, such as the "www." part, or the trailing slash if the resource in question is a directory. However, these omissions technically make it a different URL, so the web browser cannot make these adjustments, and has to rely on the server to respond with a redirect. It is possible, but due to tradition rare, for a web server to serve two different pages for URLs that differ only in a trailing slash.

Note that in en.wikipedia.org/wiki/Train, the hierarchical order of the five elements is org (generic top-level domain) - wikipedia (second-level domain) - en (subdomain) - wiki - Train; i.e. before the first slash from right to left, then the rest from left to right.

For a more extensive discussion of HTTP URLs and their use, see above.

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The big picture

The term URL is also used outside the context of the World Wide Web. Database servers specify URLs as a parameter to make connections to it. Similarly any Client-Server application following a particular protocol may specify a URL format as part of its communication process.

Example of a database URL :

jdbc:datadirect:oracle://myserver:1521;sid=testdb

If a webpage is uniquely and more or less permanently defined by a URL it can be linked to (see also permalink, deep linking). This is not always the case, e.g. a menu option may change the contents of a frame within the page, without this new combination having its own URL. A webpage may also depend on temporarily stored information. If the webpage or frame has its own URL, this is not always obvious for someone who wants to link to it: the URL of a frame is not shown in the address bar of the browser, and a page without address bar may have been produced. The URL may be derivable from the source code and/or "properties" of various components of the page. See also Webpage#URL.

Apart from the purpose of linking to a page or page component, one may want to know the URL to show the component alone, and/or to lift restrictions such as a browser window without toolbars, and/or of a small non-adjustable size.

Web servers also have the ability to redirect URLs if the destination has changed, allowing sites to change their structure without affecting existing links. This process is known as URL redirection.

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See also

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External links

Retrieved from "http://en.wikipedia.org/wiki/Uniform_Resource_Locator"
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