A Uniform Resource Locator (URL), colloquially termed a web address, is a reference to a web resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identifier (URI), although many people use the two terms interchangeably.[a] URLs occur most commonly to reference web pages (http), but are also used for file transfer (ftp), email (mailto), database access (JDBC), and many other applications.
Most web browsers display the URL of a web page above the page in an address bar. A typical URL could have the form
http://www.example.com/index.html, which indicates a protocol (
http), a hostname (
www.example.com), and a file name (
Uniform Resource Locators were defined in RFC 1738 in 1994 by Sir Tim Berners-Lee, the inventor of the World Wide Web, 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 filenames. Conventions already existed where server names could be prefixed to complete file paths, preceded by a double slash (
Berners-Lee later expressed regret at the use of dots to separate the parts of the domain name within URIs, wishing he had used slashes throughout, and also said that, given the colon following the first component of a URI, the two slashes before the domain name were unnecessary.
An early (1993) draft of the HTML Specification referred to "Universal" Resource Locators. This was dropped some time between June 1994 (RFC 1630) and October 1994 (draft-ietf-uri-url-08.txt).
Every HTTP URL conforms to the syntax of a generic URI. The URI generic syntax consists of a hierarchical sequence of five components:
URI = scheme:[//authority]path[?query][#fragment]
where the authority component divides into three subcomponents:
authority = [userinfo@]host[:port]
:), consisting of a sequence of characters beginning with a letter and followed by any combination of letters, digits, plus (
+), period (
.), or hyphen (
-). Although schemes are case-insensitive, the canonical form is lowercase and documents that specify schemes must do so with lowercase letters. Examples of popular schemes include
irc. URI schemes should be registered with the Internet Assigned Numbers Authority (IANA), although non-registered schemes are used in practice.[b]
:), followed by an at symbol (
@). Use of the format
username:passwordin the userinfo subcomponent is deprecated for security reasons. Applications should not render as clear text any data after the first colon (
:) found within a userinfo subcomponent unless the data after the colon is the empty string (indicating no password).
/). A path is always defined for a URI, though the defined path may be empty (zero length). A segment may also be empty, resulting in two consecutive slashes (
//) in the path component. A path component may resemble or map exactly to a file system path, but does not always imply a relation to one. If an authority component is present, then the path component must either be empty or begin with a slash (
/). If an authority component is absent, then the path cannot begin with an empty segment, that is with two slashes (
//), as the following characters would be interpreted as an authority component. The final segment of the path may be referred to as a 'slug'.
?), containing a query string of non-hierarchical data. Its syntax is not well defined, but by convention is most often a sequence of attribute-value pairs separated by a delimiter.
#). The fragment contains a fragment identifier providing direction to a secondary resource, such as a section heading in an article identified by the remainder of the URI. When the primary resource is an HTML document, the fragment is often an
idattribute of a specific element, and web browsers will scroll this element into view.
A web browser will usually dereference a URL by performing an HTTP request to the specified host, by default on port number 80. URLs using the
https scheme require that requests and responses will be made over a secure connection to the website.
Internet users are distributed throughout the world using a wide variety of languages and alphabets and expect to be able to create URLs in their own local alphabets. An Internationalized Resource Identifier (IRI) is a form of URL that includes Unicode characters. All modern browsers support IRIs. The parts of the URL requiring special treatment for different alphabets are the domain name and path.
The domain name in the IRI is known as an Internationalized Domain Name (IDN). Web and Internet software automatically convert the domain name into punycode usable by the Domain Name System; for example, the Chinese URL
xn-- indicates that the character was not originally ASCII.
The URL path name can also be specified by the user in the local writing system. If not already encoded, it is converted to UTF-8, and any characters not part of the basic URL character set are escaped as hexadecimal using percent-encoding; for example, the Japanese URL
http://example.com/%E5%BC%95%E3%81%8D%E5%89%B2%E3%82%8A.html. The target computer decodes the address and displays the page.
Protocol-relative links (PRL), also known as protocol-relative URLs (PRURL), are URLs that have no protocol specified. For example,
//example.com will use the protocol of the current page, either HTTP or HTTPS.
http://www.example.comis a URL, while
.0portions of dot-decimal notation to be dropped or raw integer IP addresses to be used.
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