Uniform Resource Locator

A Uniform Resource Locator, URL (pronounced as "earl" (SAMPA: [@rl]) or spelled out), or web address, is a standardized address for some resource (such as a document or image) on the Internet. First created by Tim Berners-Lee for use on the World Wide Web, the currently used forms are detailed by IETF standard RFC 2396 (1998).

The URL was a fundamental innovation in creating the World Wide Web It combines into one simple address the four basic items of information necessary to find a document anywhere on the Internet:

• The protocol to use to communicate with that machine
• The machine or domain name to go to
• An open network port on the target machine connected to some service
• The path or file name on that machine

A typical simple URL can look like:

  " class="external">http://www.wikipedia.org:80/wiki

where

• http specifies which protocol to use.
• //www.wikipedia.org specifies the domain name to contact.
• 80 specifies the network port number of the remote machine. Under most circumstances, this portion may be omitted entirely. In the case of the http protocol the default value is 80.
• /wiki is the request path on the specified system.

Most Web browsers do not require the user to enter "http://" to go to a web page. One usually just enters the page name (without the slashes) such as www.wikipedia.org/wiki/Train. To go to the homepage one usually just enters the domain name such as www.wikipedia.org

HTTP URLs can also contain additional elements, like a query string (placed after the path and separated from it by a question mark (?)) containing information from a HTML form with method=get, or a name tag (placed after the path and separated from it by a sharp mark (#)) giving the location within a hypertext page to display. FTP URLs often contain a port number.

examples:

http://www.wikipedia.org/w/wiki.phtml?title=Train&action=history
" class="external">http://www.wikipedia.org/wiki/Train#Model_railways

URLs are one type of URI.

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 defined by a URL it can be linked to (see also 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.

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.

See also Uniform Resource Identifier, website, internet, History of the Internet

For Wikipedia, see Wikipedia:URLs.