Linux NAT
This is a short howto for setting up Network Address Translation on a linux system (Debian was used for this howto)
Manually enable forwarding
$echo 1 > /proc/sys/net/ipv4/ip_forward
Enable forwarding aumatically at boot time
add the following in /etc/network/options
ip_forward=yes
Set up source NAT
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE(replace eth0 by the interface connecting your Debian router to the Internet)

Discussion