IPV6_V6ONLY

new socket option in RFC3493 (aka RFC2553bis)
restricts AF_INET6 sockets to IPv6 communications only
Linux
w/ ipv4-mapped address
allows both sockets bound to same port number if ipv6 socket is set IPV6_V6ONLY.
ipv6 sucks ipv4 traffic if ipv4 socket does not set IPV6_V6ONLY.