< World-Wide Web > (URL, previously "Universal") A standard way of specifying the location of an object, typically a web page , on the Internet . Other types of object are described below. URLs are the form of address used on the World-Wide Web . They are used in HTML documents to specify the target of a hyperlink which is often another HTML document (possibly stored on another computer).
Here are some example URLs:
http://www.w3.org/default.html http://www.acme.co.uk:8080/images/map.gif http://www.foldoc.org/?Uniform+Resource+Locator http://www.w3.org/default.html#Introduction ftp://wuarchive.wustl.edu/mirrors/msdos/graphics/gifkit.zip ftp://spy:secret@ftp.acme.com/pub/topsecret/weapon.tgz mailto:fred@doc.ic.ac.uk news:alt.hypertext telnet://dra.com
The part before the first colon specifies the access scheme or protocol . Commonly implemented schemes include: ftp , http (World-Wide Web), gopher or WAIS . The "file" scheme should only be used to refer to a file on the same host. Other less commonly used schemes include news , telnet or mailto ( e-mail ).
The part after the colon is interpreted according to the access scheme. In general, two slashes after the colon introduce a hostname (host:port is also valid, or for FTP user:passwd@host or user@host). The port number is usually omitted and defaults to the standard port for the scheme, e.g. port 80 for HTTP.
For an HTTP or FTP URL the next part is a pathname which is usually related to the pathname of a file on the server. The file can contain any type of data but only certain types are interpreted directly by most browsers . These include HTML and images in gif or jpeg format. The file's type is given by a MIME type in the HTTP headers returned by the server, e.g. "text/html", "image/gif", and is usually also indicated by its filename extension . A file whose type is not recognised directly by the browser may be passed to an external "viewer" application , e.g. a sound player.
The last (optional) part of the URL may be a query string preceded by "?" or a "fragment identifier" preceded by "#". The later indicates a particular position within the specified document.
Only alphanumerics, reserved characters (:/?#" <> %+) used for their reserved purposes and "$", "-", "_", ".", "&", "+" are safe and may be transmitted unencoded. Other characters are encoded as a "%" followed by two hexadecimal digits. Space may also be encoded as "+". Standard SGML "& ;" character entity encodings (e.g. "é") are also accepted when URLs are embedded in HTML. The terminating semicolon may be omitted if & is followed by a non-letter character.
The authoritative W3C URL specification .
(2000-02-17)