The IPv4 address an IPv6 address stands for, if it embeds one that routing
would reach: IPv4-mapped (::ffff:a.b.c.d), the NAT64 well-known prefix
(64:ff9b::/96) and 6to4 (2002:aabb:ccdd::/48). Checking only the IPv6 form
would let ::ffff:127.0.0.1 through as "some IPv6 address".
The IPv4 address an IPv6 address stands for, if it embeds one that routing would reach: IPv4-mapped (::ffff:a.b.c.d), the NAT64 well-known prefix (64:ff9b::/96) and 6to4 (2002:aabb:ccdd::/48). Checking only the IPv6 form would let ::ffff:127.0.0.1 through as "some IPv6 address".