An IPv4 application running on a dual-mode stack can communicate with an IPv4 partner. The source and destination addresses are native IPv4 addresses and the packet is an IPv4 packet.

If a partner is IPv6 enabled and running on an IPv6-only stack, then communication fails. The partner has only a native IPv6 address (not an IPv4-mapped IPv6 address). The native IPv6 address for the partner cannot be converted into a form that the AF_INET application understands.

Figure 1. IPv4-only application on a dual-mode stack