Which port state indicates the port is accessible and responds to probes, but there is no application listening on it?

Which port state indicates the port is accessible and responds to probes, but there is no application listening on it?

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

Answer: (A)

When a port is reachable and responds to probes but no application is listening on it, that is the closed state. This means the target can be contacted and acknowledges the probe, but there’s no service listening on that port to accept connections. This differs from open, where a service is actually listening; from filtered, where a firewall blocks or drops probes so you get no usable response; from unfiltered, where the port is reachable but it’s not yet determined if it’s open or closed; and from open|filtered, which indicates an ambiguous result between open and filtered.

When a port is reachable and responds to probes but no application is listening on it, that is the closed state. This means the target can be contacted and acknowledges the probe, but there’s no service listening on that port to accept connections. This differs from open, where a service is actually listening; from filtered, where a firewall blocks or drops probes so you get no usable response; from unfiltered, where the port is reachable but it’s not yet determined if it’s open or closed; and from open|filtered, which indicates an ambiguous result between open and filtered.