| Uniform Resource Locator (URL, previously "Universal") A way of specifying the location of an object, typically a , on the . Other types of object are described below. URLs are the form of address used on the . They are used in documents to specify the target of a 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 . Commonly implemented schemes include: , (World-Wide Web), or . The "file" scheme should only be used to refer to a file on the same host. Other less commonly used schemes include , or mailto (). The part after the colon is interpreted according to the access scheme. In general, two slashes after the colon introduce a (host:port is also valid, or for user:passwd@host or user@host). The 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 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 . These include and images in or format. The file's type is given by a type in the HTTP headers returned by the server, e.g. "text/html", "image/gif", and is usually also indicated by its . A file whose type is not recognised directly by the browser may be passed to an external "viewer" , 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 digits. Space may also be encoded as "+". Standard "&;" 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 (http://www.w3.org/hypertext/WWW/Addressing/Addressing.html)}. (2000-02-17) |
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