The only range size supported currently is /16 which makes some sense for IPv4, but not so much for IPv6. By default, $wgBlockCIDRLimit['IPv6'] = 19 so a range of /19 should also be used as the default size for IPv6 blocks.
|Stalled||None||T179459 Enhance blocking in AbuseFilter|
|Stalled||None||T179454 AbuseFilter range blocks should be smarter|
|Resolved||Huji||T179456 AbuseFilter should use a different range size for IPv6 range blocks|