The Web is good example of a two-tier client-server architecture (Figure 2-10). Each client computer needs an application layer package called a Web browser. There are many different browsers, such as Microsoft’s Internet Explorer. Each server on the network that will act as a Web server needs an application layer software package called a Web server. There are many different Web servers, such as those produced by Microsoft and Apache.
To get a page from the Web, the user must type the Internet uniform resource locator(URL) for the page he or she wants (e.g.,www.yahoo.com) or click on link that provides the URL. The UHL specifies the Internet address of the Web server and the directory and name of the specified, the Web server will
Provide whatever page has been defined as site’s home page.
For the requests from the Web browser to be understood by the Web server, they must use the same standard protocol or language. If there were no standard and each Web browser used a different protocol to request pages, then it would impossible for a Microsoft Web browser to communicate with an Apache Web server, for example.
The standard protocol for communication between a Web browser and a Web server is Hypertext Transfer Protocol (HTTP). To get a page from a web server, the Web browser issues a special packet called an HTTP request that contains the URL and other information about the Web page requested (see Figure 2-10). Once the server receives the HTTP request
it processes it and sends back an HTTP request, will be the requested page an error message (see Figure2-10).
This request-response dialogue occurs for every file transferred between the client and the server. For example, suppose the client requests a Web page that two graphic images. Graphics are stored in separate files format than the HTML used for the Web page (in JPEG [Joint Photographic Experts Group] format, for example). In this case, there would be three request-response pairs. First, the browser would issue a request for the Web page, and the server would send the response. Then, the browser would begin displaying the Web page and notice the two graphic files. The browser would then send a request for the first graphic and a request for the second graphic, and the server would reply with separate HTTP responses, one for each request.