mandrake's internet connection sharing can be a bit patchy, to say the least. i've tried their 'wizard' with 10.0 and 10.1, and in both cases, it screwed up.
to cut a long story short, mandrake's implementation is way OTT for a simple home network - it installs a firewall (necessary), and also a proxy server (squid) and dns, too i think...
the firewall is necessary to do the ip masquerading used to share your connection, but i've found that mandrake's implementation is screwy. so, to start with, you need to uninstall the shorewall firewall, and then also disable squid at startup (in services in mandrake control center).. then go to the shorewall website and download and install a proper version of the software. there's also excellent documentation on the site which should help you get things working nicely. it sounds complicated, but in fact it isn't - all you need is shorewall set up correctly and it'll work great.