There is no way to hide your external (ISP) address without going through an anonymous proxy (and most of thse are now mapped by the Feds - National Security, doncha know).
If you are looking to protect your home/business machines, then a firewall running NAT is your best bet.
The external address is your ISP address, the internal address should be one of the RFC addresses (192.168.X.X, 172.16-31.X.X, 10.X.X.X). These addresses are not routed through the Internet (the first router to see 'em filters 'em)...so you can't be attacked directly (though there are ways to get through..but it protects you against most of the twerps).
Good Luck
Scott