Network Address Translation, the mainstay of the IPv4 world.
Setting up NAT in Mac OS X
(snarfed from Davi's node for my easy reference)
# natd -dynamic -interface ppp0 -v & # for verbose output natd -dynamic -interface ppp0 ipfw add divert natd ip from any to any via ppp0 sysctl -w net.inet.ip.forwarding=1
