Clients and servers establish connections and communicate via sockets. Connections are communication links that are created over the Internet using TCP. Some client/server applications are also built around the connectionless UDP. These applications also use sockets to communicate.

Sockets are the endpoints of Internet communication. Clients create client sockets and connect them to server sockets. Sockets are associated with a host address and a port address. The host address is the IP address of the host where the client or server program is located. The port address is the communication port used by the client or server program. Server programs use the well-known port number associated with their application protocol.

A client communicates with a server by establishing a connection to the socket of the server. The client and server then exchange data over the connection. Connection-oriented communication is more reliable than connectionless communication because the underlying TCP provides message-acknowledgment, error-detection, and error-recovery services.

When a connectionless protocol is used, the client and server communicate by sending datagrams to each other's socket. The UDP is used for connectionless protocols. It does not support reliable communication like TCP.