Use as regras abaixo só navega o IP que voce determinar

modprobe iptable_nat
echo 1 > /proc/sys/net/ipv4/ip_forward

insmod ip_conntrack_ftp
insmod ip_nat_ftp

iptables -t nat -A POSTROUTING -s 10.0.0.10 -o eth0 -j MASQUERADE
iptables -t nat -A POSTROUTING -s 10.0.0.20 -o eth0 -j MASQUERADE
iptables -t nat -A POSTROUTING -s 10.5.2.41 -o eth0 -j MASQUERADE