$address() returns the following when handling IPv6 addresses, for example, take /mode $active +b $address($nick,3).
You are very likely to find less hostname usage as IPv6 support widens since this will require reverse delegation support for customers, which isn't likely to happen with most ISPs.
$address examples:
0: *!as@2601:a:f00:1f:dddd:c2df:f194:8003
1: *!*as@2601:a:f00:1f:dddd:c2df:f194:8003
2: *!*@2601:a:f00:1f:dddd:c2df:f194:8003
3: *!*as@2601:a:f00:1f:dddd:c2df:f194:*
4: *!*@2601:a:f00:1f:dddd:c2df:f194:*
5: A-KO!as@2601:a:f00:1f:dddd:c2df:f194:8003
6: A-KO!*as@2601:a:f00:1f:dddd:c2df:f194:8003
7: A-KO!*@2601:a:f00:1f:dddd:c2df:f194:8003
8: A-KO!*as@2601:a:f00:1f:dddd:c2df:f194:*
9: A-KO!*@2601:a:f00:1f:dddd:c2df:f194:*
Notice how each of these addresses would not result in a proper ban for the user. The structure of an IPv6 address is 64-bits network, 64-bits host. The host generation portion can potentially be a dynamically rotated, "temporary" IPv6 address, as the address above is.
Please consider adding the ability to ban based on Network Mask/CIDR. Many IRC Servers already support this functionality in both v4 and v6. Banning based on CIDR has been tested on Efnet and Freenode.
An example of a CIDR ban above that would satisfy the banning requirements would be:
/mode #channel +b *@2601:a:f00:1f::/64
On IPv4, a similar ban would be the following:
/mode #channel +b *@10.100.24.0/24
Defaulting to /64 should cover the majority of users, as it is likely they wouldn't be able to request a new network prefix from their ISP very easily as it would require changing routers. More advanced users may use Linux/OSS platforms that allow changing their DUID (the id used to pull a prefix from an ISP).
CIDR banning could be similar to the following:
$address($nick,bitmask)
$address(A-KO,64) -> *@2601:a:f00:1f::/64
$address(A-KO,56) -> *@2601:a:f00:1f::/56
$address(A-KO,48) -> *@2601:a:f00::/48
