URLs and URIs

The need to locate documents on the Internet in a standard way led to the development of the URL (Uniform Resource Locator) scheme. This scheme can also be used to identify resources on a local intranet, network or local computer. URLs are used in Web browsers, to find the home page of a Web site, then to navigate around the site. In XML, URLs are used to locate entities and now, as in HTML, to create hypertext links between documents (see Chapter 27). They also play a role in XInclude (Chapter 12), and in the Namespaces standard (Chapter 10). Strictly speaking, the URL scheme is just part of a wider scheme, known as the URI (Uniform Resource Identifier) standard. There have been several attempts to define URIs, such as RFC 2396 (1989), which can be found at www.ietf.org/rfc/rfc2396.txt. As the precise meaning of 'URI' and 'URL' has changed, the W3C has released a note that clarifies this (www.w3.org/TR/2001/NOTE-uri-clarification-20010921). The URL part of the URI standard is used to access files at a given physical location, generally using a file specification that includes at least a file name, but may also include a path to the file, as well as the method to be used to access it.

Although the term 'resource' is used in the standard, often this means a data file, or data that can be generated as the result of a search query. XML documents, DTDs, stylesheets, images and other required resources are usually stored as files.