I was actually wondering about using netmask 0xffffffff for the external interface. As you noted, they are different networks, I just wanted to be able to use any 192.168/16 ip address in the internal network and use nat-to and rdr-to in Computer1 so every packet going to or from the ISP router comes from or goes to 192.168.1.10 (and block everything else).
But still, that (external connections) is the last thing I am going to test. At the moment not even a ping from two directly connected computers that are actually sending and receiving the packets (according to tcpdump in both computers) seems to work...
