Please follow the below mentioned procedure orderwise.. (to be run at your Firewall box)
Code:
echo 1 >> /proc/sys/net/ipv4/ip_forward
then
Code:
iptables -t nat -A POSTROUTING -o eth0(device-connected-to-internet) -j MASQUERADE
OR if you have a static ip; then
Code:
iptables -t nat -A POSTROUTING -s Your-lan-series -j SNAT --to x.x.x.x(your public static-ip)
Now configure your lan-clients to have this firewall box as their gateway & now you can ping public ip addresses from your clients. But i do not suggest you to have this setup as all those spam/spywares would now be easily n freely communicate to their hosts. And if you still persist to have this kinda setup only; i would rather suggest you to have a fully configured firewall at your firewall box instead of just the code i have provided you above.
Assumptions;
1. You are not having any other firewall script at your box.
2. You have iptables installed at your firewall box.
3. You are able to browse & ping public ips from your firewall box.