You might also have a look at hosts.allow and hosts.deny (http://linux.about.com/od/commands/l/blcmdl5_hostsal.htm is just the first google result; the man pages certainly have more info, but I don't use hosts.* myself so I can only really provide a pointer). I'm not sure that really adds anything that the firewall rule wouldn't already, though.
well, if i understood the question correctly, this should do. put to file /etc/hosts.allow: ALL:ALL put to file /etc/hosts.deny:sshd: .your.domain.com allowed_ip_addresses allowed_networks allowed_hostnames
you can put more or less anything on the line and control who's allowed to connect (man hosts.deny). i'd say it is straightforward and works immediatelly without a need to (re)configure a firewall.
best, -- Lubos _@_" http://www.lubos.vrbka.net