Assume there are two internet users (A and B) that connect to the internet from the same gateway by using NAT. That means they use same IP address while passing through our firewall. Now assume we detect illegal traffic (such as DDoS) from this gateway and the attacker is user A.
In this case, is it possible to block only user A so that innocent user B can continue to connect to servers behind our firewall? Surely we can not achieve this by IP blocking. If possible, which options do I have to do that?
To clarify, I added a network topology.