I want to setup my ubuntu computer to route packets. Therefore I've read several tutorials which always had many rules to add to iptables.
I finally found out, that the only rule needed (given that iptables is otherwise completely empty) to route packets is the following:
iptables -t nat -A POSTROUTING -o <ext> -j MASQUERADE
ext is the network interface where packets are going outwards.
I don't really understand what this rule really does and why it only takes this single rule to make IP forwarding work.