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What does that indicate in the case that if a scan packet receives no response in the situation that an ethical hacker is ACK-scanning against a network segment he knows is sitting behind a stateful firewall?

A The port is filtered at the firewall.

A stateful firewall will not allow an ACK packet to pass unless it was "sourced" from within the network. The lack of response indicates that the firewall screened that port packet and did not allow it to pass.