A computer language has a set of instructions for mathematical operations, a set of instructions for string manipulation, a set of instructions for input/ output access, and so on. If we need a process to be able to communicate with another process, we need a new set of instructions to tell the lowest four layers of the TCP/IP suite to open the connection, send and receive data from the other end, and close the connection. A set of instructions of this kind is normally referred to as Application Programming Interface (API).