try this
ppp0=Internet connection
eth0=local area network connection
This will forward port 22 on the Internet to machine 192.168.0.2 port 22
on local network.
iptables -A FORWARD -p tcp -i ppp0 -o eth0 -d 192.168.0.2 --dport 22 -j
ACCEPT
iptables -t nat -A PREROUTING -p tcp -i ppp0 --dport 22 -j DNAT
--to-destination 192.168.0.2:22