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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP World Wide Web (WWW, "The Web") and the Hypertext Transfer Protocol (HTTP)
                     9  TCP/IP World Wide Web and Hypertext Overview and Concepts

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World Wide Web Media and the Hypertext Markup Language (HTML)
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TCP/IP Hypertext Transfer Protocol (HTTP)
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World Wide Web Addressing: HTTP Uniform Resource Locators (URLs)
(Page 2 of 3)

HTTP URL Syntax

HTTP URLs may be absolute or relative. Absolute URLs are usually used for hyperlinks from one Web site to another, or by users requesting a new document without any prior context. Absolute HTTP URLs are based on the following common Internet URL syntax:

<scheme>://<user>:<password>@<host>:<port>/<url-path>;<params>?<query>#<fragment>

For the Web, the scheme is “http:” and the semantics of the different URL elements is defined to have meanings that are relevant to the Web. The general structure of an HTTP URL is thus:

http://<user>:<password>@<host>:<port>/<url-path>?<query>#<bookmark>

The following shows how these syntactic elements are specifically defined for HTTP absolute URLs:

  • <user> and <password>: Optional authentication information, for resources located on password-protected servers. This construct is rarely used in practice and so most people don't realize it is an option; it has thus become a target of abuse by con artists who use it to obscure undesirable URLs.

  • <host>: The host name of the Web server upon which the resource is located. This is usually a fully-qualified DNS domain name, but may also be an IP address.

  • <port>: The TCP port number to use for connecting to the Web server. This defaults to 80 for HTTP and is usually omitted. Rarely, you may see some other port number used, sometimes to allow two copies of Web server software devoted to different uses on the same IP address; port 8080 is especially common as an alternative.

  • <url-path>: The path pointing to the specific resource to be retrieved using HTTP. This is usually a full directory path expressing the sequence of directories to be traversed from the root directory to the place where the resource is located, and then the resource's name. It’s important to remember that the path is case-sensitive, even though DNS domain names are not.

  • <query>: An optional query or other information to be passed to the Web server. This feature is commonly used to implement interactive functions, because the query value can be specified by the user and then be passed from the Web browser to the Web server. The alternative method is by using the HTTP POST method.

  • <bookmark>: Identifies a particular location within an HTML document. This is commonly used in very large HTML documents to allow a user to click a hyperlink and scroll to a particular place in the document. See the example in the previous topic (near the end of Table 273).

Although the URL syntax for the Web is quite “rich” and potentially complex, most Web URLs are actually quite short. The vast majority of these components are omitted, especially the user, password, port and bookmark elements; queries are also used only for special purposes. This leaves the more simplified form you will usually encounter for URLs:

http://<host>/<url-path>

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Version 3.0 - Version Date: September 20, 2005

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