Whenever you use an ATM to check your balance or withdraw some money, you are actually participating in a client-server interaction with a computer on your bank’s network. A client-server network consists of at least two computers: one that runs a program requesting services and another that processes those requests. An ATM is actually a client programmed to connect you with your bank's servers. Once a link is established with the ATM, programs on the bank's servers process and fulfill your request for cash or balance information. Server Sockets Client and server programs need to be able to establish connections to each other over the network. Client and server applications connect through methods defined by Transmission Control Protocol/Internet Protocol -- or TCP/IP -- standards. The server is linked or bound to a specific IP address on the network that is reachable by the client. The server also has a TCP port number that identifies the type of service it provides. For instance, a client program looking for TCP ports 20 or 21 needs File Transfer Protocol services. Like an electrical socket and plug, the server and client programs connect and data begins to flow between them. Client Application Functions Once a connection is established, the client sends the server whatever data and instructions are needed to complete the tasks it needs the server to perform. The client application takes the data it receives from the server, displays it, saves it to a storage device, prints it or forwards it to other clients on the network. Server Role The server program plays a passive role: it constantly checks its network connection to see if there are any clients sending it requests for a service it can provide. Once the server program detects a service request, it establishes the socket and begins receiving and sending information to the client until the job is done. Finally, the server closes the connection to the client and resumes the wait for more client program requests for services. Example of Client Server Operation Whenever you click on a Web page link or use a search engine, you are initiating a client-server program interaction. Your Web browser is a complex client program that connects to a Web server. The Web server sits waiting for a client to ask for a Web page. Once it detects a request, it establishes the socket, locates the Web page requested and sends it over the Internet to the user's browser.