TCP/IP Tutorial

As you learned in Hour 16, "The Internet: A Closer Look," the view of the Web page that you see through the window of your Web browser is the result of a conversation between the browser and a Web server computer. The language used for that conversation is called Hypertext Transfer Protocol (HTTP). The data that is delivered from the server to the client is a finely crafted jumble of text, images, addresses, and formatting codes rendered to a unified document through an amazing versatile formatting language called Hypertext Markup Language (HTML).

HTML is conceptually similar to a word processing format. In fact, the best way to begin a discussion of HTML is to consider how word processing documents evolved. Experts have always recognized the need to store and transmit information written in a human language (such as English or Russian or French). Strategies quickly evolved for efficiently storing and displaying alphanumeric characters. In the United States, the ASCII standard coded each letter and number (and many punctuation signs) into a single bit pattern. ASCII text files were used throughout the computing world for configuration files, online help documents, and electronic mail messages. Text files are still an important feature on Unix/Linux operating systems. At some point, the rapidly evolving computer technology began to merge with the rapidly evolving word processing technology. For professional printed documents, vendors needed a way to introduce formatting into the text file. Was it possible to make a heading appear in boldface, change a margin, or change to a different font? The vendors developed numerous systems (many of them proprietary) for coding formatting information into a text document. Some of these systems used ASCII-based codes. Others used different digital markers to denote formatting information.

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Of course, these formatting code systems work only if the application that writes the document and the application that reads the document agree on what each code means.

These word processing systems became increasingly sophisticated. Some developed the capability to reference another file, such as a figure, that would then appear in the text when the document was displayed.

The creators of HTML wanted to develop a universal, vendor-neutral system for encoding format information. They wanted to include not just typesetting codes but also pictures and layout information. And they added another innovation that was to become a powerful and important feature of the new format: the hypertext link.

A link is a segment of text, or even just a region of the screen, that causes the browser to open a new page or move to a different part of the page. Links let the reader view the online information in small doses. The reader can choose whether or not to link to another page for additional information. HTML documents can be assembled into unified systems of pages and links (see Figure 17.1). A visitor can find a different path through the data depending on how the visitor traverses the links. And the Web developer has almost unlimited ability to define where a link will lead. The link can lead to another HTML document in the same directory, a document in a different directory, or even a document on a different computer. The link might lead to a totally different Web site on another computer across the world.

Within the HTML code, the link takes the form of a special address called a uniform resource locator (URL). The form of URL most associated with the Web is

The URL specifies the protocol to use to access a resource and the DNS name of the Web server. This example is readily identified by anyone who has ever worked with a Web browser. It is also common to see a path and filename appended to the URL:

The URL can actually transmit a number of additional parameters. You sometimes see a long, complex URL with additional parameters in the address box of the browser window after you access a site through one of the Internet search engines. You'll learn more about the general form of URLs in the next section.

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You might have noticed that the URL for a typical Web site (such as www.whitehouse.gov) includes only a DNS domain name and doesn't seem to make reference to a filename. If the filename is not specified, the browser automatically opens a default filename defined by the Web server.

A Web browser navigates by URLs. You access a Web page by entering the URL of the page in the address box of the browser window (see Figure 17.2). When you click on a link, the browser opens the Web page specified in the link's URL.

To summarize this brief introduction, an HTML document contains some combination of

To visit a Web site, the user enters the URL of the Web site into the Web browser window. The browser initiates a connection to the Web server specified in the URL. The server sends the HTML data across the network to the Web browser. The Web browser assembles the HTML data into the view of the Web page that appears in the browser window. The following sections discuss this process in greater detail. Of course, the process has recently gotten complicated by new features such as scripting and Dynamic HTML. You'll learn about these new features also.

What Is the World Wide Web?

Figure 17.1. A Web site is a unified system of pages and links.

Figure 17.2. Enter the URL in the address box of the browser window.