Uniform resource locator
A uniform resource locator, 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.
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 slashes are used to separate directory and file names. Conventions already existed where server names could be prepended to complete file paths, preceded by a double-slash (//).
Berners-Lee later regretted the use of dots to separate the parts of the domain name within URIs, wishing he had used slashes throughout. For example, http://www.example.com/path/to/name would have been written http:com/example/www/path/to/name. Berners-Lee has also said that, given the colon following the URI scheme, the two forward slashes before the domain name were also unnecessary.
Every URL consists of the following: the scheme name (commonly called protocol), followed by a colon, two slashes,[note 1] then, depending on scheme, a server name (exp. ftp., www., smtp., etc.) followed by a dot (.) then a domain name[note 2] (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:
- 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:email@example.com may start an e-mail composer with the address firstname.lastname@example.org 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://email@example.com. Passwords embedded in this way are not conducive to secure working, but the full possible syntax is
- The domain name or IP address gives the destination location for the URL.
- The domain google.com, or its IP address 18.104.22.168, 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
- 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.
List of allowed URL characters 
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 - _ . ~
Have to be encoded sometimes
! * ' ( ) ; : @ & = + $ , / ? % # [ ]
URLs as locators 
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 file:// scheme) may both be represented using a URL, but may additionally provide a protocol (particularly useful for remote items) and credentials.
See also 
- CURIE (Compact URI)
- Fragment identifier
- Internationalized Resource Identifier (IRI)
- URL normalization
- Clean URL
- Berners-Lee has said that, given the colon following the URI scheme, the two slashes before the domain name were unnecessary.
- Berners-Lee also later regretted the use of dots to separate the parts of the domain name, wishing he had used slashes throughout. For example, http://www.example.com/path/to/name would have been written http:com/example/www/path/to/name
- RFC 3305 "URI Partitioning: There is some confusion in the web community over the partitioning of URI space, specifically, the relationship among the concepts of URL, URN, and URI. The confusion owes to the incompatibility between two different views of URI partitioning; the 'classical' and 'contemporary' views."
- RFC 1738 Uniform Resource Locators (URL). This RFC is now obselete. It has been superseded by a newer RFC (see the RFC Index)
- "Living Documents BoF Minutes". W3.org. Retrieved 2011-12-26.
- "URL Specification". Retrieved 2011-12-26.
- Berners-Lee, Tim. "Frequently asked questions by the press". Retrieved 2010-02-03.
- "Technology | Berners-Lee 'sorry' for slashes". BBC News. 2009-10-14. Retrieved 2010-02-14.
- RFC 1738
- "PHP parse_url() Function". Retrieved 2009-03-12.
- "URL Syntax". Pangea.stanford.edu. 2004-07-20. Retrieved 2011-12-26.
- "RFC 3986: Uniform Resource Identifier (URI): Generic Syntax". Network Working group of the IETF. 2005-01. p. 40. Retrieved 2013-04-24. "[...] the scheme and host are case-insensitive [...] The other generic syntax components are assumed to be case-sensitive unless specifically defined otherwise by the scheme [...]"
- Tim Berners-Lee, Roy T. Fielding, Larry Masinter. (January 2005). "Uniform Resource Identifier (URI): Generic Syntax". Internet Society. RFC 3986; STD 66.
- "Deprecated NSOPenPanel Methods". Apple Inc. Retrieved 7 October 2012.