Browsers and Web Servers

So far you have learned how computers connect to one another so that they can exchange information. Once the connection is established, the next stage is for data to flow between the two computers.

In most situations, the computer that initiates the exchange is called the client, and the computer that receives the connection is called the server. A computer program runs on the server at all times, listening for connections from clients. On the client computer, another program (such as a web browser) connects to the server whenever it requires information. For example, when you request a Web page, the browser makes a connection to the Web server for that page when you click the Go button.

In a typical client/server scenario, the client sends some data called a request to the server, and the server determines the nature of the request and formulates a response, which it sends back to the client. For example, when an email program reads your email from a mail server, the following steps occur:



The client sends a user name and password to the server.


The server responds to say that the user name and password are accepted.


The client requests a list of emails that are on the server.


The server responds with a list of emails, not including the body of the email.


The client requests the body of a specific email (for example when you double-click on the email to view it).


The server sends the email body.

The client and server must have a common understanding of the contents of the request and response messages. We saw earlier how when the two computers made a network connection, they have common understanding, defined by the TCP/IP protocol, that enables them to send data and confirm that it has been delivered. On top of this network protocol sits another messaging protocol that specifies a common language for computer applications to talk to each other. The protocol between two computer applications is called an application protocol. You can imagine a network protocol like the regulations that govern how to construct a road, and an application protocol like the regulations that govern how to drive on the road.

The most common Internet application protocol is the Hypertext Transfer Protocol (HTTP), which is the main protocol used in the World Wide Web. You may also have come across the File Transfer Protocol (FTP), which you can use to transfer files from one computer to another.

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