Which port state means a port is accessible, but Nmap is unable to determine whether it is open or closed?

• A. open|filtered
• B. filtered
• C. open
• D. unfiltered

Answer: (D)

In Nmap’s port-state classification, the idea being tested is what a given state says about reachability and what can be inferred from probes. The state described as unfiltered means the port is reachable and not blocked by a firewall, but Nmap cannot determine whether the port is actually open (a service listening) or closed. So the port is accessible, yet the scan cannot decide its true status from that probe alone. This is distinct from open (service listening), closed (no service listening), and filtered (firewall blocks probes), and also from open|filtered (ambiguous between open and filtered).