Clients need to locate servers, and the simplest way to achieve this is by having the server bind to a pre-agreed address (other more complex methods also exist).

Servers do not need to know the client’s address in advance, they just send their replies to the address from which the client’s request originated. Therefore clients can (normally) bind to nearly any port number, although high port numbers in the range 0xC000 to 0xFFFF are reserved for use by FreeRTOS+TCP itself, and many low port numbers are (by convention only) reserved for common network services.

The standard echo service provides a convenient example. Echo servers simply echo back the data sent to them by clients. Echo servers use port 7 by convention. The sequence diagram below shows a socket being created and bound on both an echo client and an echo server, and then a single echo transaction between the client and the server (it should be noted that the sockets only need to be created and bound once, not for each transaction).