This article is incomplete and was first written in September 2006 for the BeezNest technical website (http://glasnost.beeznest.org/articles/343).
An example on how to activate NAT on a Debian box (on Etch and up).
First, activate IP forwarding in /etc/sysctl.conf.
Then, you need to activate NAT itself in /etc/network/interfaces:
auto eth0 iface eth0 inet static address 192.168.1.254 netmask 255.255.255.0 network 192.168.1.255 up iptables -t nat -A POSTROUTING -o $IFACE -j MASQUERADE
There you are.