Comment by icedchai

Yes, but NAT combined with RFC1918 private addresses does provide a layer of security. This is the most common NAT configuration for 99.99% of residential users. It is what most people mean by "NAT."

If your address cannot be routed across the Internet, it can't be accessed, firewall or not.

I have worked in corporate environments where we NAT'd public, route-able addresses for historical reasons. That would be insecure without a firewall and is not what most people are discussing.