the falcon would have it sown private address .. say 192.168.1.123 the mac will be on the same subment as the rest of your pc's which are physically on ethernet say 192.168.0.xxx. You need to ammended the routing table on the mac to forward the requestes from 192.168.1.123 to your asl gateway/router/proxy.
for the lan to "see" the falcon you would need to tell either each machine about the route via the mac or have a central routing table.
to add the route under unix/windows u issue a "route" command or if u have a dhcp/wins server they should be able to spit out the required info.
