nection to sends queries to a Web server. A server is an application program that accepts connection requests and responds to queries. An HTTP request identifies the resource (URL) that the client is inter- ested in and tells the server what to do with it. HTTP enables users to fetch different kinds of resources, such as text, picture, and audio. For HTTP a TCP connection is first established and requests and replies are sent through this connection. The URL identifies, with the help of DNS, the destination host, and the server port used for HTTP is 80. HTTP requests contain information about what a server should do with the resource, the identification of the actual resource or page, and the HTTP version in use. HTTP replies consist of a header containing informa- tion about the resource, such as file type (how the browser should handle file), and actual resource or document. The standard known as Multipurpose Internet Mail Extensions (MIMEs) specifies content types included in responses to tell to browser how to deal with it. Figure 6.56 illustrates the procedure. The steps that occur between the user's click and the page being dis- played are as follows: 1. The browser determines the URL http://www.w3.org/public/courses/ internet. html that is written or pointed out with a mouse. 2. The browser asks the DNS for the IP address of www.w3.org. Data Communications TCP connection establishment HTTP request Get/public/courses/internet.html HTTP reply internet.html Client, A new HTTP request browser Server HTTP reply TCP port TCP port 80 selected when for HTTP TCP connection is established www.w3.org 18.23.0.23 Figure 6.56 Procedure for HTTP requests and replies. 3. DNS replies with 18.23.0.23. 4. The browser establishes a TCP connection to port 80 on 18.23.0.23. 5. The browser sends a GET/public/courses/internet.ht HTTP/1.0 command. 6. The server www.w3.org replies with the file internet. html. 7. The TCP connection is released. 8. The browser displays all the text in internet. html. 9. The browser fetches and displ