Networking noob here. I want to prevent all incoming requests except through a specific port, and that traffic is forwarded to a specific device on the network. NAT seems to do that just fine, it’s almost like a kind of firewall by itself. What kind of threats are there that requires more than just NAT for security?
NAT is not a firewall. NAT does not inspect packet payloads, it doesn’t do anything except attempt to route packets to where they are supposed to go. If the connection originates from outside or it is a ‘connectionless’ protocol, the NAT has no idea which internal IP to route to, so it drops the packet.
NAT provides some security by sheer coincidence and not by design.