URL

A Uniform Resource Locator is a Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. In popular usage, technical documents and verbal discussions it is often incorrectly used as a synonym for URI. The best known example of a URL is the "address" of a web page on the World Wide Web, e.g. http://htmlcss.wikia.com.

Syntax

Every URL consists of some of the following: the scheme name (commonly called protocol), followed by a colon, then, depending on scheme, a hostname (alternatively, IP address), a port number, the path of the resource to be fetched or the program to be run, then, for programs such as Common Gateway Interface (CGI) scripts, a query string, and with HTML documents, an anchor (optional) for where the page should start to be displayed.

The combined syntax is

scheme://username:password@domain:port/path?query_string#anchor

• The scheme name, or resource type, defines its namespace, purpose, and the syntax of the remaining part of the URL. Most Web-enabled programs will try to dereference a URL according to the semantics of its scheme and a context. For example, a Web browser will usually dereference the URL http://example.org:80 by performing an HTTP request to the host example.org, at the port number 80. Dereferencing the URL mailto:bob@example.com will typically start an e-mail composer with the address bob@example.com in the "To" field.
  • Other examples of scheme names include https:, gopher:, wais:, ftp:. URLs that specify https as a scheme (such as https://example.com/) denote a secure website.
• The registered domain name or IP address gives the destination location for the URL. The domain google.com, or its IP address 72.14.207.99, is the address of Google's website.
• The hostname and domain name portion of a URL are case-insensitive, since the DNS is specified to ignore case. http://en.example.org/ and HTTP://EN.EXAMPLE.ORG/ both open the same page.
• The port number is optional; if omitted, the default for the scheme is used. For example, if http://myvncserver.example.com:5800 is typed into the address bar of a browser it will connect to port 5800 of myvncserver.example.com; this port is used by the VNC remote control program and would set up a remote control session. If the port number is omitted, a browser will connect to port 80, the default HTTP port.
• The path is used to find the resource specified. It is case-sensitive, though it may be treated as case-insensitive, by some servers, especially those based on Microsoft Windows. If the server is case sensitive and http://en.example.org/wiki/URL is correct, http://en.example.org/WIKI/URL/ or http://en.example.org/wiki/url/ will display an HTTP 404 error page, unless these URLs point to a valid resource themselves.
• The query string contains data to be passed to web applications such as CGI programs. The query string contains name/value pairs separated by ampersands (&), with names and values in each pair separated by equal signs, for example first_name=John&last_name=Doe.
• The anchor part when used with HTTP specifies a location on the page. For example, http://en.example.org/wiki/URL#Syntax addresses the beginning of the "Syntax" section of the page.
  • On some sites, the anchor may have other functions; see: fragment identifier.