For example a http-Server uses port number 80, SSH uses port 22 and so on. The combination of IP-address and port number is called socket and is unique. Therefore connections are uniquely defined by their endpoints (=sockets), a connection sends data from the clients socket to the server socket and vice versa, for example from the socket with IP 188.8.131.52, port 65432 to the socket with IP 184.108.40.206, Port 80 as it may occur for a browser on 220.127.116.11 that connects to a http-server on 18.104.22.168. Server processes are usually using standardized ports, so called 'well known ports'. A client usually uses a port from the upper port range (larger than 1023).