|  | 15.4 | Application Layer Examples
| 15.4.3 | Hypertext transfer protocol
Hypertext Transfer Protocol (HTTP) works with the World Wide Web, which is the fastest growing and most used part of the Internet. One of the main reasons for the extraordinary growth of the Web is the ease in which it allows access to information. A Web browser (along with all the other network applications covered in this chapter) is a client-server application, which means that it requires both a client and a server component in order to function. A Web browser presents data in multimedia formats on Web pages that use text, graphics, sound, and video. The Web pages are created with a format language called Hypertext Markup Language (HTML). HTML directs a Web browser on a particular Web page to produce the appearance of the page in a specific manner. In addition, HTML specifies locations for the placement of text, files, and objects that are to be transferred from the Web server to the Web browser. Hyperlinks make the World Wide Web easy to navigate. A hyperlink is an object (word, phrase, or picture) on a Web page that, when clicked, transfers you to a new Web page. The Web page contains (often, hidden within its HTML description) an address location known as a Uniform Resource Locator (URL). In the following example, the "http://" tells the browser which protocol to use. The second part, "www", tells the browser what type of resource it wishes to contact. The third part, "cisco.com," identifies the domain of the Web server IP address. The last part, "edu" identifies the specific folder location (on the server) that contains the Web page. Example: http://www.cisco.com/edu/ When you open a Web browser, the first thing you usually see is a starting (or "home") page. The URL of the home page has already been stored in the configuration area of your Web browser and can be changed at any time. From the starting page you can click on one of the Web page hyperlinks, or type a URL in the browser’s address bar. The Web browser then examines the protocol to determine if it needs to open another program, and determines the IP address of the Web server. After that, the transport layer, network layer, data link layer, and physical layer initiate a session with the Web server. The data that is transferred to the HTTP server contains the folder name of the Web page location. (Note: The data can also contain a specific file name for an HTML page.) If no name is given, the server uses a default name (as specified in the server’s configuration). The server responds to the request by sending all of the text, audio, video, and graphic files, as specified in the HTML instructions, to the Web client. The client browser reassembles all the files to create a view of the Web page, and then terminates the session. If you click on another page that is located on the same, or a different server, the whole process begins again.
| Web Links
| Hypertext Transfer Protocol