[goohle]

IPv4-Compatible IPv6 address are deprecated, because people can use IPv6 to circumvent IPv4 NAT firewalls, so OS vendors asked to ban this behavior. It's also confusing to people when they can call both IPv4 and IPv6 at the same time:

  $ ping 127.0.0.1
  PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
  64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.043 ms
  # OK
 
  $ ping6 ::1
  PING ::1(::1) 56 data bytes
  64 bytes from ::1: icmp_seq=1 ttl=64 time=0.063 ms
  # OK

  $ ping ::1
  PING ::1(::1) 56 data bytes
  64 bytes from ::1: icmp_seq=1 ttl=64 time=0.023 ms
  # OK

  $ ping6 ::ffff:127.0.0.1
  PING ::ffff:127.0.0.1(::ffff:127.0.0.1) 56 data bytes
  ping6: sendmsg: Invalid argument
  # CONFUSING & DANGEROUS