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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 (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 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 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/Train 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, as HTTP is by far the most common protocol used in web browsers. One usually just enters the page name such as www.wikipedia.org/wiki/Train. To go to the homepage one usually just enters the domain name such as www.wikipedia.org.
Since, using the HTTP protocol, it is possible for 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.
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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.
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 :
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.
URLs in general are case-sensitive. For some URLs and parts of URLs this is not the case.