During the transition of the Internet from IPv4 to IPv6 it is typical to operate in a mixed addressing
environment, and for this purpose a special notation has been introduced to express IPv4-mapped
and IPv4-compatible IPv6 addresses by writing the final 32 bits of an address in the familiar IPv4
dotted-quad notation. For example, the IPv4-mapped IPv6 address ::ffff:c000:0280 is usually written
as ::ffff:192.0.2.128, thus expressing clearly the original IPv4 address that was mapped to IPv6.
[] Constrained IP6Address: Recognise in opt and unapply
[] Unconstrained IP6Address: Recognise in opt and unapply
https://en.wikipedia.org/wiki/IPv6_address#Transition_from_IPv4
optandunapplyoptandunapply