How secure is NAT?

If you are running many-to-one NAT (PAT in cisco terminology, more aptly termed Network Address Port Translation (NAPT)) you are secure for attacks originating outside your environment since your NAT (NAPT) device will not know what machine to send any traffic to (unless your firewall also hosts that IP address - then you are as secure as whatever remote administration setting you have on your firewall).

If you are running any internal public servers, then you are only as secure as the protocol allowed inbound and however secure that machine's OS is.

Also, you are not protected from your (or your users') downloading malicious code, viruses, trojan horsed, etc.

Hope that helps -

NAT is essentially an address conservation technique which then indirectly offered the benefit of concealing a network's IP addresses from the outside world. What class of attacks are you talking about?? I can think of atleast five to defeat NAT. Remember, NAT is only a workaround and IMHO not a security strategy. There are a whole bunch of techniques to discover hosts hiding behind an address. I do not wish to discuss them here in any more detail. Just note, that today NAT continues to give many netadmins a false sense of security.

Thanks,
--Rookie