World Wide Web Addressing: HTTP Uniform Resource Locators (URLs)
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The main reason that hypertext is so powerful and useful is that it allows related documents to be linked together. In the case of the World Wide Web, this is done using a special set of Hypertext Markup Language (HTML) tags that specifies in one document the name of another document that is related in some important way. A user can move from one document to the next using a simple mouse click. The Web has succeeded largely on the basis of this simple and elegant method of referral.
The notion of hyperlinking has some important implications on how Web documents and other resources are addressed. Even though the Web is at its heart a message transfer protocol similar to the File Transfer Protocol (FTP), the need to be able to define hyperlinks meant that the traditional FTP model of using a set of commands to specify how to retrieve a resource had to be abandoned. Instead, a system was needed whereby a resource could be uniquely specified using a simple, compact string.
The result of this need was the definition of one of the three primarily elements of the World Wide Web: the Uniform Resource Identifier (URI). URIs are divided into two categories: Uniform Resource Locators (URLs) and Uniform Resource Names (URNs). While URIs, URLs and URNs grew out of the development of the Web, they have now been generalized to provide an addressing mechanism for a wide assortment of TCP/IP application layer protocols. They are thus described in detail in a separate section; this topic will provide more information on how they are used specifically for the Web.
At the present time, the Web uses URLs almost exclusively; URNs are still in development. Web URLs specify the use of the Hypertext Transfer Protocol (HTTP) for resource retrieval, and are thus normally called HTTP URLs. These URLs allow a resource such as a document, graphical image or multimedia file to be uniquely addressed by specifying the host name, directory path and file name where it is located.
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