To function as a server, a device must be configured to listen to requests from clients on a network connection. This functionality can exist as part of the operating system as an installed application, role, or a combination of the two.
For example, Microsoft’s Windows Server operating system provides the functionality to listen to and respond to client requests. Additionally, installed roles or services increase which kinds of client requests the server can respond to. In another example, an Apache web server responds to Internet browser requests via an additional application, Apache, installed on top of an operating system.
When a client requires data or functionality from a server, it sends a request over the network. The server receives this request and responds with the appropriate information. This is the request and response model of client-server networking, also known as the call and response model.
A server will often perform numerous additional tasks as part of a single request and response, including verifying the identity of the request or, ensuring that the client has permission to access the data or resources requested, and properly formatting or returning the required response in an expected way.