It wouldn't work in mirc the same way it works with irc servers: irc servers typically match the CIDR mask against the users' IP address even if they have a resolved hostname, while mirc cannot just resolve every hostname before doing the matching. It's always going to be half-broken in that sense anyway (this is not specific to CIDR bans btw); I would say stuff like this should remain in the realm of scripting.

Saturn, QuakeNet staff