Uniform Resource Locator Definition URIs And URLs

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.

1 Definition

1.1 URIs and URLs

URLs are one type of URI. Historically, the terms have been used almost synonymical 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.

1.2 URL Scheme

An URL begins with the name of its scheme, followed by a colon, followed by a scheme-specific part.

1.3 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:

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.

1.4 URI References

The term URI reference means a particular instance of a URI as it is being used in, for instance an HTMLHyperText Markup Language (HTML) is a markup language designed for creating web pages, that is, information presented on the World Wide Web. Defined as a simple "application" of SGML, which is used by organizations with complex publishing requirements, HT document. It introduces two new concepts: the one of absolute and relative references, and that of a fragment identifier.

An absolute URL is just like a URL defined above. A relative URL comprises just the scheme-specific part, where the scheme is inferred from the context in which the URL reference appears (the Base URI).

A URI reference can also consist of a URI 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 the resource has been retrieved. Therefore it is never sent to the server in HTTP GET requests.

1.5 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.

2 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 protocolIn networking, a communications protocol or network protocol is the specification of a set of rules for a particular type of communication. Different protocols often describe different aspects of a single communication; taken together, these form a protoc to use to communicate,
the host ( serverThis article is about computer servers. For the food service use, see waiter. In computing, a server is: A computer software application that carries out some task on behalf of users. This is usually divided into file serving, allowing users to store and) to communicate with,
the network portIn computing, a port (derived from seaport) is usually an interface through which data is sent and received. An exception is a software port (derived from transport), which is software that has been "transported" to another computer system (see below for on the server to connect to,
the pathIn computing, the PATH is an environment variable listing directories where common executables may be found. Also, a path is the general form of a file or directory name. It intends to be a path to a specific file, where they may be many instances of that to the resource on the server (for example, its file name).

A typical URL can look like:

http://en.wikipedia.org:80/wiki/Special:Search?search=train&go=Go

where

http is the protocol,
en.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/Special:Search is the resource path,
?search=train&go=Go is the query string; this part is optional.

Most web browserA web browser is a software package that enables a user to display and interact with documents hosted by web servers. Popular browsers include Microsoft Internet Explorer and Mozilla Firefox. A browser is the most commonly used kind of user agent. The lars do not require the user to enter "http://" to address a webpageA website or web page is a "page" of the World Wide Web, usually in HTML format (the file extensions are typically htm or html) and with hypertext links to enable navigation from one page or section to another. Webpages often use associated graphics files, 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.

3 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 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.

4 See also

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