How it works...

The scan results of filtered and unfiltered ports depends on whether a firewall being used is stateful or stateless. A stateful firewall checks if an incoming ACK packet is part of an existing connection or not. It blocks it if the packets are not part of any requested connection. Hence, the port will show up as filtered during a scan.

Whereas, in the case of a stateless firewall, it will not block the ACK packets and the ports will show up as unfiltered.