I use proftp
are all the allowable IPs in the same subnet?
if so, you could use something like:
iptables -A INPUT -p tcp -s x.x.x.x/x -i eth0 -dport 21 -j ACCEPT
iptables -A INPUT -p udp -s x.x.x.x/x -i eth0 -dport 21 -j ACCEPT
where x.x.x.x/x is the subnet/size and eth0 is to be replaced with your outward facing interface
if not, you'd have to do:
iptables -A INPUT -p tcp -s x.x.x.x -i eth0 -dport 21 -j ACCEPT
iptables -A INPUT -p udp -s x.x.x.x -i eth0 -dport 21 -j ACCEPT
for each allowable IP address, where x.x.x.x is the IP to allow and eth0 is to be replaced with your outward facing IP address
there's a good tutorial here:
http://www.yolinux.com/TUTORIALS/Lin...rkGateway.html
It has a simple setup for dropping everything apart from the connections you want, but doesn't allow for dropping everything not from specific IPs. A combination of what I've said and what's there should get you going.