What is the cost in performance for all of these NAT features? Not many hard numbers are available. For NT's ICS, performance is probably a moot point, since it has to involve a dial-up interface. Certainly ICS will function fast enough to max out a dial-up connection. IP Masquerade could have some meaningful testing done to it, but I'm not aware of any performance testing that has been done. In addition, Linux is very much a moving target. Changes come quickly, and they may include performance enhancements. Linux also runs on a wide variety of platforms, so if you run into a performance bottleneck while using IP Masquerade, you'll probably be able to scale it up with better hardware. Cisco has provided some rough numbers here:

http://www.cisco.com/warp/public/458/41.html#Q6

Cisco gives numbers for three of their router platforms: 4500, 4700, and 7500. The 4500 is able to run at about 7.5 8.0 Mbps on 10Mb Ethernet for all packet sizes. The 4700 is able to run at 10 Mbps on 10Mb Ethernet for all packet sizes. The 7500 throughput ranges from 24 Mbps for 64-byte packets, to 96 Mbps for 1500-byte packets on Fast Ethernet.

Of course, for all three NAT packages we've been looking at, this depends on what else these platforms are doing. If the NT ICS server is running a CPU-intensive game at the time, performance may dip. If the Cisco router is also performing an encryption on the traffic, performance will drop there, too.