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A uniform resource locator (abbreviated URL; also known as a web address, particularly when used with HTTP) is a specific character string that constitutes a reference to a resource. Most web browsers display the URL of a web page above the page in an address bar. A typical URL might look like:
RFC 3986 (2005) classifies URLs as a specific type of uniform resource identifier (URI), although many people use the two terms interchangeably. A URL implies the means to access an indicated resource, which is not true of every URI. URLs occur most commonly to reference web pages (http), but can also have a role in file transfer (ftp), email (mailto), database access (JDBC), and many other applications (see URI scheme for a list).
The Uniform Resource Locator was standardized 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 slashes before the domain name were also unnecessary.
Every HTTP URL consists of the following, in the given order. Several schemes other than HTTP also share this general format, with some variation.
The scheme says how to connect, the host specifies where to connect, and the remainder specifies what to ask for.
The syntax is:
google.com, or its numeric IP address
188.8.131.52, is the address of Google's website.
HTTP://EN.EXAMPLE.ORG/both open the same page.
http://vnc.example.com:5800connects 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.
http://en.example.org/wiki/URLis correct, then
http://en.example.org/wiki/urlwill display an HTTP 404 error page, unless these URLs point to valid resources themselves.
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.
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 security, but the full possible syntax is
Other schemes do not follow the HTTP pattern. For example, the
mailto scheme only uses valid email addresses. When clicked on in an application, the URL
mailto:firstname.lastname@example.org may start an e-mail composer with the address
email@example.com in the To field. The
tel scheme is even more different; it uses the public switched telephone network for addressing, instead of domain names representing Internet hosts.
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:
! * ' ( ) ; : @ & = + $ , / ? % # [ ]
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.
The protocol, or scheme, of a URL defines how the resource will be obtained. Two common protocols on the web are HTTP and HTTPS. For various reasons, many sites have been switching to permitting access through both the HTTP and HTTPS protocols. Each protocol has advantages and disadvantages, including for some users that one or the other protocol either does not function, or is very undesirable. When a link contains a protocol specifier it results in the browser following the link using the specified protocol regardless of the potential desires of the user. It is possible to construct valid URLs without specifying a protocol which are called protocol-relative links (PRL) or protocol-relative URLs. Using PRLs on a page permits the viewer of the page to visit new pages using whichever protocol was used to obtain the page containing the link. This supports continuing to use whichever protocol the viewer has chosen to use for obtaining the current page when accessing new pages.
An example of a PRL is //en.wikipedia.org/wiki/Main_Page which is created by removing the protocol prefix.
Internet users are distributed throughout the world using a wide variety of languages and alphabets. Users expect to be able to create URLs in their own local alphabets.
Internationalized Resource Identifiers (IRIs) are a form of URL which include most Unicode characters. All modern browsers support IRIs. The parts of the URL requiring special treatment for different alphabets are the domain name and path.  
For example the Chinese web site
becomes the following for DNS lookup. The xn-- indicates the character was not originally ASCII. 
The URL path name can also be specified by the user in their local alphabet. It is then converted to Unicode, if it is not already in Unicode, and then any characters which are not part of the basic URL character set are converted to English letters using percent-encoding.
For example the following Japanese Web page
The target computer can then convert the page name back to Unicode and display the appropriate page. 
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.
http://www.example.com/path/to/namewould have been written
The term "Uniform Resource Locator" (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network "location").
[...] 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 [...]