Uniform Resource Locator (URL)

A uniform resource locator (URL), abbreviated URL, also known as web address, is a specific character string that constitutes a reference to a resource. In most web browsers, the URL of a web page is displayed on top inside an address bar. An example of a typical URL would be "http://en.example.org/wiki/Main_Page". A URL is technically a type of uniform resource identifier (URI), but in many technical documents and verbal discussions, URL is often used as a synonym for URI.

Every URL consists of the following: the scheme name (commonly called protocol), followed by a colon, two slashes, then, depending on scheme, a server name (exp. ftp., www., smtp., etc.) followed by a dot (.) then a domain name (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 an optional fragment identifier.

The syntax is: scheme://domain:port/path?query_string#fragment_id

The scheme name defines the namespace, purpose, and the syntax of the remaining part of the URL. Software will try to process a URL according to its scheme and context. For example, a web browser will usually dereference the URL http://example.org:80 by performing an HTTP request to the host at example.org, using  port number 80. The URL mailto:bob@example.com may 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 with https as a scheme (such as https://example.com/) require that requests and responses will be made over a secure connection to the website. Some schemes that require authentication allow a username, and perhaps a password too, to be embedded in the URL, for example ftp://asmith@ftp.example.org. Passwords embedded in this way are not conducive to secure working, but the full possible syntax is
scheme://username:password@domain:port/path?query_string#fragment_id

The 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 domain name portion of a URL is not case sensitive since DNS ignores 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, http://vnc.example.com:5800 connects to port 5800 of vnc.example.com, which may be appropriate for a VNC remote control session. If the port number is omitted for an http: URL, the browser will connect on port 80, the default HTTP port. The default port for an https: request is 443.

The path is used to specify and perhaps find the resource requested. 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, then 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 valid resources themselves.

The query string contains data to be passed to software running on the server. It may contain name/value pairs separated by ampersands, for example  ?first_name=John&last_name=Doe.

The fragment identifier, if present, specifies a part or a position within the overall resource or document. When used with HTML, it usually specifies a section or location within the page, and used in combination with Anchor Tags the browser is scrolled to display that part of the page.