There's a very nice file in FreeBSD called /etc/login.access. This is used to say who can log on on the machine and from where. I'll link you to a very well made tutorial by Michael Lucas. I'm seriously thinking that all UNIX OSes should have a file like that. I emailed the NetBSD people to ask them if they intended to include something like that.

http://www.onlamp.com/pub/a/bsd/2001...y_Daemons.html