Auniform resource locator (URL), colloquially known as anaddress on theWeb,[1] is a reference to aresource that specifies its location on acomputer network and a mechanism for retrieving it. A URL is a specific type ofUniform Resource Identifier (URI),[2][3] although many people use the two terms interchangeably.[4][a] URLs occur most commonly to referenceweb pages (HTTP/HTTPS) but are also used for file transfer (FTP), email (mailto), database access (JDBC), and many other applications.
Mostweb browsers display the URL of a web page above the page in anaddress bar. A typical URL could have the formhttp://www.example.com/index.html, which indicates a protocol (http), ahostname (www.example.com), and a file name (index.html).
The format combines the pre-existing system ofdomain names (created in 1985) withfile path syntax, whereslashes are used to separatedirectory andfilenames. Conventions already existed where server names could be prefixed to complete file paths, preceded by a double slash (//).[9]
Berners-Lee later expressed regret at the use of dots to separate the parts of thedomain name withinURIs, wishing he had used slashes throughout,[9] and also said that, given the colon following the first component of a URI, the two slashes before the domain name were unnecessary.[10]
EarlyWorldWideWeb collaborators including Berners-Lee originally proposed the use of UDIs: Universal Document Identifiers.An early (1993) draft of the HTML Specification[11] referred to "Universal" Resource Locators. This was dropped some time between June 1994 (RFC 1630) and October 1994 (draft-ietf-uri-url-08.txt).[12] In his bookWeaving the Web, Berners-Lee emphasizes his preference for the original inclusion of "universal" in the expansion rather than the word "uniform", to which it was later changed, and he gives a brief account of the contention that led to the change.
Every HTTP URL conforms to the syntax of a generic URI. The URI generic syntax consists of fivecomponents organized hierarchically in order of decreasing significance from left to right:[13]: §3
A component isundefined if it has an associated delimiter and the delimiter does not appear in the URI; the scheme and path components are always defined.[13]: §5.2.1 A component isempty if it has no characters; the scheme component is always non-empty.[13]: §3
A non-emptyscheme component followed by a colon (:), 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 includehttp,https,ftp,mailto,file,data andirc. URI schemes should be registered with theInternet Assigned Numbers Authority (IANA), although non-registered schemes are used in practice.[b]
An optionalauthority component preceded by two slashes (//), comprising:
An optionaluserinfo subcomponent followed by an at symbol (@), that may consist of auser name and an optionalpassword preceded by a colon (:). Use of the formatusername:password in 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).
Ahost subcomponent, consisting of either a registered name (including but not limited to ahostname) or anIP address.IPv4 addresses must be indot-decimal notation, andIPv6 addresses must be enclosed in brackets ([]).[13]: §3.2.2 [c]
An optionalport subcomponent preceded by a colon (:), consisting of decimal digits.
Apath component, consisting of a sequence of path segments separated by a slash (/). 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 afile system path but does not always imply a relation to one. If an authority component is defined, then the path component must either be empty or begin with a slash (/). If an authority component is undefined, then the path cannot begin with an empty segment—that is, with two slashes (//)—since the following characters would be interpreted as an authority component.[16]: §3.3
By convention, inhttp andhttps URIs, the last part of apath is namedpathinfo and it is optional. It is composed by zero or more path segments that do not refer to an existing physical resource name (e.g. a file, an internal module program or an executable program) but to a logical part (e.g. a command or a qualifier part) that has to be passed separately to the first part of the path that identifies an executable module or program managed by aweb server; this is often used to select dynamic content (a document, etc.) or to tailor it as requested (see also:CGI and PATH_INFO, etc.).
where:"/questions" is the first part of thepath (an executable module or program) and"/3456/my-document" is the second part of thepath namedpathinfo, which is passed to the executable module or program named"/questions" to select the requested document.
Anhttp orhttps URI containing apathinfo part without aquery part may also be referred to as a 'clean URL,' whose last part may be a 'slug.'
An optionalquery component preceded by a question mark (?), consisting of aquery string of non-hierarchical data. Its syntax is not well defined, but by convention is most often a sequence ofattribute–value pairs separated by adelimiter.
An optionalfragment component preceded by ahash (#). The fragment contains afragment 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 anHTML document, the fragment is often anid attribute of a specific element, and web browsers will scroll this element into view.
A web browser will usuallydereference a URL by performing anHTTP request to the specified host, by default on port number 80. URLs using thehttps scheme require that requests and responses be made over asecure connection to the website.
Internationalized URL
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. AnInternationalized Resource Identifier (IRI) is a form of URL that includesUnicode characters. All modern browsers support IRIs. The parts of the URL requiring special treatment for different alphabets are the domain name and path.[18][19]
The domain name in the IRI is known as anInternationalized Domain Name (IDN). Web and Internet software automatically convert the domain name intopunycode usable by theDomain Name System; for example, the Chinese URLhttp://例子.卷筒纸 becomeshttp://xn--fsqu00a.xn--3lr804guic/. Thexn-- indicates that the character was not originallyASCII.[20]
The URL path name can also be specified by the user in the local writing system. If not already encoded, it is converted toUTF-8, and any characters not part of the basic URL character set are escaped ashexadecimal usingpercent-encoding; for example, the Japanese URL http://example.com/引き割り.html becomeshttp://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.[18]
Protocol-relative URLs
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, typically HTTP or HTTPS.[21][22]
^A URL implies the means to access an indicated resource and is denoted by a protocol or an access mechanism, which is not true of every URI.[5][4] Thushttp://www.example.com is a URL, whilewww.example.com is not.[6]
^The procedures for registering new URI schemes were originally defined in 1999 byRFC2717, and are now defined byRFC 7595, published in June 2015.[14]
^For URIs relating to resources on the World Wide Web, some web browsers allow.0 portions of dot-decimal notation to be dropped or raw integer IP addresses to be used.[15]
^HistoricRFC1866 (obsoleted byRFC 2854) encourages CGI authors to support ';' in addition to '&'.[17]: §8.2.1
