The Uniform Resource Locator (URL) is the standard method of specifying the location (or address) of an object (or resource) on the Internet. The URL for a particular site on the World Wide Web (WWW or Web) can be compared to a postal address for a particular physical location. Typical objects located on Web pages include information and documents, and these objects usually also transport users to other pages on the Web. (In the past URL was the acronym for Universal Resource Locator, but has been generally replaced by the term Uniform Resource Locator.) URLs are used to specify the protocol (a standard procedure for regulating data transmission between computers) to be used. For example, URLs are used in the Web's protocol, called HTTP (HyperText Transfer Protocol), where the resource can be an HTML (HyperText Markup Language) page, an image file, a program such as a CGI (Common Gateway Interface) application or Java applet (a small program sent along with a Web page), or any other file supported by HTTP. The resource can further specify the target of a hyperlink, often another HTML document stored on another computer system.
People who "surf" the Web use URLs to access resources by typing a string of characters that identifies the type of document, the computer the document is on, the directories and subdirectories the document is in, and the name of the document. To move within the Web, type the URL in the "address bar" on the Web browser, and press "enter" to begin surfing the Internet. For example, the URL of the home Web page for the United States's Library of Congress (LOC) is http://www.loc.gov/help/about.html.
Generally, the URL address is made up of these parts: protocol://host.domainname/directory/filename.filetype. The part of the URL before the colon represents the access protocol (the scheme, or set of rules for transporting and retrieving information); http means the document is on the WWW. But not all URLs use the HTTP protocol. Other protocols are possible, including (1) ftp (File Transfer Protocol) that allows the user to list files on, retrieve files from, and add files to another computer on the Internet, (2) gopher that consists of a menu-driven document delivery system for retrieving information from the Internet, (3) news that involves documents on a Usenet newsgroup (a forum for users to post and respond to message), (4) Telnet that is an access method in which users log on to a remote computer, (5) mailto (electronic mail, or e-mail), and (6) WAIS (Wide Area Information Server).
In general, two slashes after the colon introduces a host.domainname. Thus, the next part of the URL, in this case www.loc.gov, represents the specific computer (server) on which the document (file) is found: that is, www is the host and loc.gov is the domainname. (As a note, most people commonly use "URL" and "domain name" interchangeably.) The .gov extension identifies the computer as belonging to the United States government. Some other common extensions are: .com (commercial), .net (networking), .mil (military), .org (organization), and .edu (education, usually a college or university). A URL does not always have to be written with the complete address. If a user is already surfing within http://, for instance, then that portion can be left off. For example, if a user wishes to go from the Library of Congress Web site to the White House Web site, and both reside in http://, then simply type in www.whitehouse.gov. This format is considered a "relative" or "partial" URL.
Following the host.domainname is the directory or path on which the document is found. In the example above, the directory is help, which takes one from the Library of Congress's home page to the "Help and FAQs: General Information about the Library of Congress" page. The last item listed is the document filename.filetype, which in the example is about.html. This takes one to the "The Library of Congress: About this site" page. The filename is about, and the filetype is html. Filetype often includes text documents in html format, and images in gif and jpg format.
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