After the bind, the server has established a specific IP address and port upon which other TCP/IP hosts can request connection. Now it must notify the TCP/IP address space that it intends to listen for connections on this socket. The server does this with the LISTEN3 call, which puts the socket into passive open mode. Passive open mode describes a socket that can accept connection requests, but cannot be used for communication. A passive open socket is used by a listener program like the CICS® IMS™ Listener to await connection requests. Sockets that are directly used for communication between client and server are known as active open sockets. In passive open mode, the socket is open for client contacts; it also establishes a backlog queue of pending connections.

This LISTEN call tells the TCP⁄IP address space that the server is ready to begin accepting connections. Normally, only the number of requests specified by the BACKLOG parameter will be queued.