If Nmap reports a host's port state as open, what does that imply, and how would you interpret closed and filtered states?

If Nmap reports a host's port state as open, what does that imply, and how would you interpret closed and filtered states?

• A. Open ports accept connections and have services; closed ports are reachable but have no service; filtered indicates firewall or filtering.
• B. Open ports mean the port is closed to all traffic.
• C. Closed ports always respond with ICMP unreachable.
• D. Filtered means the port is open but noisy.

Answer: (A)

Port state shows what Nmap can infer about a port based on its probes. When a port is open, a service is listening there and can accept connections, so the port will respond to connection attempts. A closed port is reachable but has no service listening on it, typically replying to probes with a reset on TCP or with an ICMP unreachable in some UDP cases. Filtered means a firewall or filtering device blocks the probes, so Nmap can’t determine whether the port is open or closed. This is why the described interpretation—open ports hosting services, closed ports being reachable but without a service, and filtered ports being blocked by filtering—best matches how Nmap reports port states.

Port state shows what Nmap can infer about a port based on its probes. When a port is open, a service is listening there and can accept connections, so the port will respond to connection attempts. A closed port is reachable but has no service listening on it, typically replying to probes with a reset on TCP or with an ICMP unreachable in some UDP cases. Filtered means a firewall or filtering device blocks the probes, so Nmap can’t determine whether the port is open or closed. This is why the described interpretation—open ports hosting services, closed ports being reachable but without a service, and filtered ports being blocked by filtering—best matches how Nmap reports port states.