Many IPv6 TCP/IP stacks support both IPv4 and IPv6 interfaces and are capable of receiving and sending IPv4 and IPv6 packets over the corresponding interfaces. These TCP/IP stacks are generally referred to as dual-mode stacks. This does not indicate that there are two separate TCP/IP stacks running on such a node, but it does indicate that the TCP/IP stack has built-in support for both IPv4 and IPv6.

A dual-mode stack supports AF_INET and AF_INET6 socket applications. AF_INET applications can communicate using IPv4 addresses. IPv6-enabled applications that use AF_INET6 sockets can communicate using both IPv6 addresses and IPv4 addresses (using the IPv4-mapped IPv6 address format).