Uniform Resource Locator

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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 (http://www.ietf.org/rfc/rfc2396.txt) (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 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 simple URL can look like:

   http://www.wikipedia.org:80/wiki/Train

where

http is the protocol.
www.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/Train is the resource path.

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 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, however it is up to the server administrator to decide to respect case when responding to requests. For convenience most webservers send the same page for URLs differing only in case.

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