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This chapter is from the book

About URIs

There are a lot of resources on the Internet: text, images, animations, video clips, audio files, and executable programs.

A URI, or uniform resource identifier, is a string of characters that identifies such a resource. Examples of a URI would include identifiers of FTP, gopher, Web, e-mail, news, and telnet services. URIs consist of the following elements:

  • The means in which the information is accessed—This is typically some kind of Internet protocol, such as FTP or HTTP.

  • The network location where the information resides—Specifically, the hostname of the resource host.

  • The path and name of the file in question—Sometimes this won't appear in the URI as a specific document because a default document or script, such as index.html, is assumed.


The familiar term URL, or uniform resource locator, defines a subset of URI that points to a specific location. Another subset of URI is the URN (uniform resource name). When describing a Web address, people will use the term URI and URL interchangeably.

Table 3.1 provides a look at an identifier for a variety of resource types.

Table 3.1  URI Schemes


URI Example













As you can see in the listings, each identifier begins with the protocol, and is followed by the location of the resource.

Managing URIs in your Web documents is a fairly easy process. The main thing to keep in mind is that on certain systems, path names and filenames may be case sensitive. So, http://www.molly.com/books/index.html might not necessarily be the same resource as http://www.molly.com/BOOKS/index.html. It largely depends on how the specific server is being managed. To properly author URIs, you'll need to be sure of the URI's case.


Because XHTML is case-specific and asks that element and attribute names are all lowercase, many people become confused and think that attribute values must also be in lowercase. However, attribute values in XHTML are not case-sensitive and can therefore accommodate mixed-case URIs.

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