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 the history of the Internet. 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:

   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. Sometimes, and also in this case, "www." can be omitted: wikipedia.org.

Note that in www.wikipedia.org/wiki/Train the hierarchical order of the five elements is org - wikipedia - www - wiki - Train, i.e. before the first slash from right to left, then the rest from left to right.

Table of contents

GET: Query Strings

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
 http://www.wikipedia.org/wiki/Train#Model_railways

The Big Picture

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. 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.

Case-sensitivity

URLs in general are case-sensitive. For some URLs and parts of URLs this is not the case.