The PORT statement reserves a port for the use of a particular server. The statement does not typically distinguish between IPv4 and IPv6; the port is reserved regardless of which type of address the application uses.

Use the BIND keyword on the PORT statement to force a listener that binds to the IPv4 INADDR_ANY address, or to the IPv6 unspecified address (in6addr_any), to listen on a particular IP address. If you specify an IPv4 address on the BIND keyword, listeners that are bound to the INADDR_ANY address are converted to the specified IPv4 address, and listeners that are bound to the in6addr_any address are converted to the IPv4-mapped IPv6 form of the specified address. If you specify an IPv6 address on the BIND keyword, the address is ignored for IPv4 listeners that are bound to the INADDR_ANY address, and listeners that are bound to the in6addr_any address are converted to the specified IPv6 address.

If you use the BIND option, your server can listen for either IPv4 connections or IPv6 connections, but not both. To have the same service serve both IPv4 and IPv6 clients, you might need to start two instances of it, one bound to an IPv4 address and one to an IPv6 address.

With SHAREPORT or SHAREPORTWLM keyword, you can start multiple instances of the server and have connections automatically load balanced between them. This function is supported for TCP listeners only. All IPv4 connection requests are load balanced between the set of IPv4 listeners (including AF_INET6 listeners bound to the IPv6 unspecified address in6addr_any), while all IPv6 connection requests are load balanced between the set of IPv6 listeners. See z/OS Communications Server: IP Configuration Reference for information about the load balancing algorithms used by each of these parameters.