Linux Network Address Translation (NAT) Service

Variables
    iif: your intranet interface
    iip: your intranet IP
    wif: your internet (WAN) interface
    wip: your wan internet (WAN) IP
    inm: your intranet netmask


Enable Forwarding
    echo 1 > /proc/sys/net/ipv4/ip_forward

Enable IP Forwarding

1.      iptables -I FORWARD -i $wif -o $iif -j ACCEPT
2.      iptables -I FORWARD -i $iif -o $wif -j ACCEPT
3.      iptables -t nat -A POSTROUTING -s $iip/$inm -o $wif -j MASQUERADE

               OR 

     iptables -t nat -I POSTROUTING -s $iip/$inm -j SNAT --to $wip