TCP is a peer-to-peer, connection-oriented protocol. There are no master/slave relationships. The applications, however, typically use a client/server model for communications.

A server is an application that offers a service to internet users; a client is a requester of a service. An application consists of both a server and a client part, which can run on the same or on different systems. Users usually invoke the client part of the application, which builds a request for a particular service and sends it to the server part of the application using TCP/IP as a transport vehicle.

The server is a program that receives a request, performs the required service and sends back the results in a reply. A server can usually deal with multiple requests and multiple requesting clients at the same time. Most servers wait for requests at a well-known port so that their clients know which port (and in turn, which application) they must direct their requests.

The client typically uses an arbitrary port called an ephemeral port for its communication. Clients that wish to communicate with a server that does not use a well-known port must have another mechanism for learning to which port they must address their requests. This mechanism might employ a registration service such as portmap, which does use a well-known port.