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THE BUILDING BLOCKS OF THE WEB
Last month we looked at the hardware infrastructure of the Internet and how computers all over the world can use it to share information. This month’s column concludes our brief Internet primer with a look at how the ubiquitous World Wide Web works.

The World Wide Web has become synonymous with the Internet. The Web, however, is just one facet of the Internet. Other services that use the Internet infrastructure described in last month’s column include electronic mail, Usenet newsgroups, Internet relay chat and Telnet, among others.

The Web makes use of a networking protocol (a protocol is a set of rules that govern the exchange of information between computers) called Hypertext Transfer Protocol, or HTTP, to transfer documents from Internet servers to computers all over the world. Whenever a user calls up a Web page with a Web browser, the browser sends a request, via HTTP, to the Web server that hosts that page and then downloads and displays the contents of the page.

How does the browser know where to send a request for a particular page? Every page on the Web has a unique Uniform Resource Locator, or URL, that allows Web browsers to find the page on a Web server. The URL is the address of the Web page.

The URL consists of a domain name followed by a file path. For instance, the URL for JADA on ADA.org is "www.ada.org/adapco/jada/j-menu.html". This address tells the browser to request and display the file named "j-menu.html" that is stored in a directory named "jada," which resides in the "adapco" directory on the "ada.org" Web server.

The documents that are transferred over the Web using HTTP are coded using Hypertext Markup Language, or HTML. A Web site is a collection of linked HTML documents. In its most basic form, an HTML document is a simple text file that is coded with tags that tell Web browsers how to display information and graphics. For example, here is a line of HTML code:

<CENTER><B><I>This is a Web page</I></B></CENTER>

A browser such as Internet Explorer or Netscape would interpret this line of code and display the text "This is a Web page" in bold italics and centered on the page.

Similarly, HTML tags are used to indicate how graphic files are displayed on a Web page and to establish hyperlinks. Hyperlinks allow users to jump to other pages on the Web with a click of the mouse. Clicking on a hyperlink commands the Web browser to request, download and display the page that is identified, by its URL, in the HTML tag for the hyperlink.

Simple HTML documents are often augmented with other components such as scripts and databases to provide the rich features found at sites such as ADA.org. Scripts are just small chunks of computer code that instruct a Web browser to perform functions beyond those indicated by HTML tags. Databases contain large quantities of data that can be dynamically displayed on Web pages according to input from users.

At its foundations, though, the Web is built on HTTP and HTML. With these basic building blocks, the Web has mushroomed into a community serving hundreds of millions of people worldwide in less than a decade. Imagine what the future must hold.

Joe Hoyle, electronic media editor, ADA Publishing, a division of ADA Business Enterprises, Inc., "hoylej{at}ada.org".

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