Guus, Yes you can share a modem with your other machines. Go to www.linuxforum.com And look for "Sharing an Internet Connection Using NAT/IPTABLES" It's very simple to setup. I have two windows and a linux laptop sharing a dialup modem attached to an linux box with a modem attached to a serial port.
In the above reference be sure to use "ppp0" in place od "eth0" in the script line starting with "$IPTABLES".
regards, Jim
