Ports

A port is a 16-bit integer that defines a specific application, within an IP address, in which several applications use the same network interface. The port number is a qualifier that TCP⁄IP uses to route incoming data to a specific application within an IP address. Some port numbers are reserved for particular applications and are called well-known ports, such as Port 23, which is the well-known port for Telnet.

As an example, an MVS™ system with an IP address of 129.9.12.7 might have CICS®, IMS™ as port 2000, and Telnet as port 23. In this example, a client desiring connection to CICS, IMS would issue a CONNECT call, requesting port 2000 at IP address 129.9.12.7.

Note: It is important to understand the difference between a socket and a port. TCP/IP defines a port to represent a certain process on a certain machine (network interface). A port represents the location of one process in a host that can have many processes. A bound socket represents a specific port and the IP address of its host. In the case of CICS, the Listener has a listening socket which has a port to receive incoming connection requests. When a connection request is received, the Listener creates a new socket representing the endpoint of this connection and passes it to the applications by way of the givesocket/takesocket calls.