IPv6 is activated on a system with a second NETWORK statement for DOMAINNAME(AF_INET6) with DOMAINNUMBER(19), which arrives at the PFS as a second vfs_network call. If a PFS supports IPv6, it must support both AF_INET and AF_INET6; there are no IPv6-only stacks.

An administrator can add the second NETWORK statement for AF_INET6 dynamically with SETOMVS RESET=. The stack is free to reject the vfs_network if it arrives after initialization. Generally, both vfs_network calls are passed to the PFS during z/OS UNIX startup or after a PFS recycles. The vfs_network calls for AF_INET and AF_INET6 may be in any order.

If PfsiIpv6 has not been set, or if the vfs_network for AF_INET6 is not accepted, IPv6 sockets are not opened to that stack. When an application opens an AF_INET6 socket across a Common INET configuration of both IPv6-capable and IPv4-only stacks, an AF_INET socket is opened to the IPv4-only stacks, and a certain amount of address conversion and emulation is performed by CINET for the IPv4-only stack. An IPv6-capable stack must do its own conversions and emulations for any IPv4 partners that it permits on an IPv6 socket.