IPv4-Mapped IPv6 Address

IPv4-Mapped IPv6 Address
This type of address also embeds an IPv4 address in the low-order 32-bits, but with 0s in only
the first 80 high-order bits and 1s in the next 16 bits—bits 81 to 96. This address type is used
by devices that support both IPv4 and IPv6 protocol stacks in order that they may communicate
with devices that support only IPv4. On the dual-stack device, an IPv6 application that
is sending traffic to the IPv4 device’s IPv4-mapped IPv6 address will recognize the meaning
of this type of address and send IPv4 packets—not IPv6 packets—to that destination. In other
words, this type of addressing mechanism does not encapsulate IPv6 packets within IPv4
packets. Conversely, if such a node receives a pure IPv4 packet that must be forwarded into
the IPv6 domain, the dual-stack node will create the IPv4-mapped IPv6 address, to be used as
the IPv6-header source address, from the incoming packet’s original IPv4 source address. So
any return traffic will be known by the dual-stack node to be destined for an IPv4-only interface,
and will be forwarded as such. IPv4-mapped IPv6 addresses are even more of a transition
mechanism, and their address format is ::FFFF:A.B.C.D, where A.B.C.D is the IPv4 unicast
address. A common use for this type of address is when an IPv6-enabled DNS server responds
to the request of a dual IPv6/IPv4 node with the IP address of an IPv4-only node. The DNS
server returns the IPv4-mapped IPv6 address, and the dual node knows what to do from there.