This will create fake IPv4 for IPv6 users based on the 64 first bits of their IP. As most currently exising IPv6 providers are assigning /64 classes to their customers, banning the generated IPv4 effectively bans the whole IPv6.

Also I do generate a 32bits IPv4 from 64bits of IPv6 using XOR. While this means people using IPv6 might share the same generated IPv4 (quite unlikely), it is usually impossible for someone to obtain a different generated IPv4 without access to more than a /64 (I believe only system administrators have this kind of thing).

You might want to store in database generated IPv4 and their IPv6 counterparts to be able to recover a given IPv6 from a blocked IPv4.

This method also allows to block IPs in the same subnet from IPv4 subnets (I don’t know if IPB supports this feature) and recognize people from the same subnet as the start of their generated ip will be the same (however whois information for the given IP will not match the real user’s ip).

To have the generated IPv4, insert this in conf_global.php :

$encoded_ip = inet_pton($_SERVER['REMOTE_ADDR']);
if (strlen($encoded_ip) == 16) {
    $ipv4 = '';
    for($i = 0; $i < 8; $i += 2) $ipv4 .= chr(ord($encoded_ip[$i]) ^ ord($encoded_ip[$i+1]));
    $_SERVER['REMOTE_ADDR'] = inet_ntop($ipv4);
}

And remember to ask your IPB support to have real IPv6 ASAP.

Searching around a bit finally got this error from Chrome:

Unsafe JavaScript attempt to access frame with URL http://bbs.gg.st/index.php?app=core&module=global&section=login from frame with URL http://www.facebook.com/extern/login_status.php?api_key=10e950be918b8f0561e2073c53f2ab8e&extern=0&channel=http%3A%2F%2Fbbs.gg.st%2Finterface%2Ffacebook%2Fxd_receiver.php&locale=en_US. Domains, protocols and ports must match.

On Firefox (and probably other browsers), this works without problem. Just sharing that so other people do not get stuck with the same problem.