Uniform Resource Locator



In computing, a uniform resource locator (URL) (originally called universal resource locator) is a specific character string that constitutes a reference to an Internet resource.

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.

History
The Uniform Resource Locator was created in 1994 by Tim Berners-Lee and the URI working group of the Internet Engineering Task Force (IETF) as an outcome of collaboration started at the IETF Living Documents "Birds of a Feather" session in 1992. The format combines the pre-existing system of domain names (created in 1985) with file path syntax, where forward slashes are used to separate folder and file names. Conventions already existed where server names could be prepended to complete file paths, preceded by a double-slash (//).

Syntax
Every URL consists of some 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 </tt>, 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/ </tt> and  HTTP://EN.EXAMPLE.ORG/ </tt> 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 </tt> 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 </tt> is correct,  http://en.example.org/WIKI/URL </tt> or  http://en.example.org/wiki/url </tt> 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</tt>.
 * The fragment identifier, if present, specifies a part or a position within the overall resource or document. When used with HTTP, it usually specifies a section or location within the page, and the browser may scroll to display that part of the page.

List of allowed URL characters
Unreserved

May be encoded but it is not necessary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9 - _. ~</tt>

Reserved

Have to be encoded sometimes

! * ' ; : @ & = + $, / ? % # [ ]</tt>

Further details can for example be found in RFC 3986 and http://www.w3.org/Addressing/URL/uri-spec.html.

URLs as locators
A URL is a URI that, in addition to identifying a resource, provides a means of locating the resource by describing its "primary access mechanism (e.g., its network location)".

Internet hostnames
On the Internet, a hostname is a domain name assigned to a host computer. This is usually a combination of the host's local name with its parent domain's name. For example, en.example.org consists of a local hostname (en) and the domain name example.org. The hostname is translated into an IP address via the local hosts file, or the domain name system (DNS) resolver. It is possible for a single host computer to have several hostnames; but generally the operating system of the host prefers to have one hostname that the host uses for itself.

Any domain name can also be a hostname, as long as the restrictions mentioned below are followed. For example, both "en.example.org" and "example.org" can be hostnames if they both have IP addresses assigned to them. The domain name "xyz.example.org" may not be a hostname if it does not have an IP address, but "aa.xyz.example.org" may still be a hostname. All hostnames are domain names, but not all domain names are hostnames.

Modern usage
Major computer manufacturers such as Apple have begun to deprecate APIs that take local paths as parameters, in favour of using URLs. This is because remote and local resources (via the scheme) may both be represented using a URL, but may additionally provide a protocol (particularly useful for remote items) and credentials.

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