Uniform Resource Locator |
A Uniform Resource Locator, URL (properly pronounced as a spelled-out initialism, not syllabalized 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 [http://www.orafaq.com/glossary/faqglosu.htm],[http://www.patrickgavin.com/SEO-Glossary.htm],[http://www.wda.org/Public/help/glossary.htm].
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.
= Definition =
== URIs and URLs ==
Every URL is a type of 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:
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.
== 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
== 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.
== 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/pathparameter=value#anchor
For another example of a HTTP URL, #URLs in everyday use.
== URI references ==
The term URI reference means a particular instance of a URI, or portion thereof, as used in, for instance, an .
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:
Examples of relative URLS:
== 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.
= 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 Communications protocol to use to communicate, *the host (server) to communicate with, *the Port (computing) on the server to connect to, *the path (computing) to the resource on the server (for example, its file name).
A typical URL can look like:
http://en.wikipedia.org:80/wiki/Special:Searchsearch=train&go=Go
where
Most 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 (computing). 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, .
= 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 .
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.
=See also=
=External links=
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