IPv6 application on a dual-mode stack

An IPv6 application on a dual-mode stack can communicate with IPv4 and IPv6 partners as long as it does not bind to a native IPv6 address. If it binds to a native IPv6 address, it cannot communicate with an IPv4 partner because the native IPv6 address cannot be converted to an IPv4 address.

If a partner is IPv6, all communication uses IPv6 packets.

If a partner is IPv4, the following results occur:

Both source and destination are IPv4-mapped IPv6 addresses.
On inbound, the transport protocol layer maps the IPv4 addresses in received IPv4 packets to their corresponding IPv4-mapped IPv6 addresses before returning to the application with AF_INET6 addresses.
On outbound, the transport protocol layer converts the IPv4-mapped IPv6 addresses to native IPv4 addresses and sends IPv4 packets.

Both an IPv6-only IP host and an IPv4-only IP host can communicate with an IPv6-enabled server. — Figure 1. IPv6 application on dual-mode stack