12.6 Summary

An IPv6 server on a dual-stack host can service both IPv4 clients and IPv6 clients . An IPv4 client still sends IPv4 datagrams to the server, but the server's protocol stack converts the client's address into an IPv4-mapped IPv6 address since the IPv6 server is dealing with IPv6 socket address structures.

Similarly, an IPv6 client on a dual-stack host can communicate with an IPv4 server. The client's resolver will return IPv4-mapped IPv6 addresses for all the server's A records, and calling connect for one of these addresses results in the dual stack sending an IPv4 SYN segment. Only a few special clients and servers need to know the protocol being used by the peer (e.g., FTP) and the IN6_IS_ADDR_V4MAPPED macro can be used to see if the peer is using IPv4.