It is likely that clients that are not connected to a link that is directly attached to a z/OS® image require access to servers that run on that z/OS image. Because z/OS provides a dual-stack implementation, z/OS can send IPv4 packets to partner nodes that are connected to the IPv4 network and IPv6 packets to partner nodes that are connected to the IPv6 network. If the client node is connected to the same routing infrastructure as the z/OS node, traffic is routed between z/OS and the client node by way of the native network transport.

In some cases, the two nodes might not be connected to the same routing infrastructure. For instance, each node might be attached to distinct IPv6 networks that are separated by an intermediate IPv4 network. When this occurs, tunneling might be used to transmit the native IPv6 packets across the IPv4 network. This allows nodes in the disjoint IPv6 networks to send packets to one another.

z/OS does not support functioning as an endpoint for this type of tunnel. However, z/OS might route traffic over a tunnel in the intermediate network. In this case, the tunnel endpoint used by z/OS would be an IPv6/IPv4 router in the network that supports one of several tunneling protocols. The tunnel endpoint used by z/OS might be attached to the same LAN to which z/OS attaches or might be attached to a remote network link. In either case, the presence of the tunnel endpoint is transparent to z/OS; from the z/OS perspective, traffic is routed over the native IPv6 network.