transfer IPv6 to IPv4 via socat
Suppose that you have an ipv6-only server A with ip [ipv6-A], and ipv4-ipv6 server B with ip (ipv4-B, [ipv6-B]).
If you wanna visit http://[ipv6-A]:80, but your ISV does not provide some ipv6 addresses. Now then, you'll make B be your "ipv6-ipv4 relay server" with following guide.
root@B:~# socat tcp4-listen:2222,fork tcp6:[2001:19f0:7001:5a34:5400:01ff:feb5:5bcd]:80
root@B:~# socat udp4-listen:2222,fork udp6:[2001:19f0:7001:5a34:5400:01ff:feb5:5bcd]:80
Now you can visit http://ipv4-B:2222 to access http://[ipv6-A]:80