Define the standard port-state terms open, closed, filtered, unfiltered, and open|filtered as reported by Nmap.

Define the standard port-state terms open, closed, filtered, unfiltered, and open|filtered as reported by Nmap.

• A. Open: the port has a listening service; Closed: the port is reachable but no service; Filtered: a firewall blocks or drops the probe; Unfiltered: the port is reachable but cannot be determined as open or closed; Open|Filtered: uncertain.
• B. Open: the port is open and a service can be contacted; Closed: the port is reachable but no service; Filtered: a firewall blocks or drops the probe; Unfiltered: the port is reachable but state ambiguous; Open|Filtered: state cannot be confirmed as open or closed.
• C. Open: the port responds with a reset; Closed: the port is reachable but listening; Filtered: ICMP unreachable; Unfiltered: the port is not reachable.
• D. Open: host unreachable; Closed: host reachable; Filtered: timeouts; Open|Filtered: definitive.

Answer: (B)

Nmap’s port-state terms describe what the tool can infer from its probes, not the actual service status alone. Open means a listening service is reachable on that port and can be contacted. Closed means the host is reachable but nothing is listening on that port. Filtered means something like a firewall or filter is blocking or dropping probes, so there’s no definitive indication of open or closed. Unfiltered means the port is reachable and not filtered, but the state (open or closed) can’t be determined from the response. Open|filtered means the port could be open or it could be filtered; there isn’t enough information to distinguish between those two possibilities. This is why the best description aligns with option describing those exact meanings. Other descriptions mix up what the responses indicate—for example, implying the port opens only when there’s a reset or mischaracterizing host reachability—so they don’t fit the standard definitions.

Nmap’s port-state terms describe what the tool can infer from its probes, not the actual service status alone. Open means a listening service is reachable on that port and can be contacted. Closed means the host is reachable but nothing is listening on that port. Filtered means something like a firewall or filter is blocking or dropping probes, so there’s no definitive indication of open or closed. Unfiltered means the port is reachable and not filtered, but the state (open or closed) can’t be determined from the response. Open|filtered means the port could be open or it could be filtered; there isn’t enough information to distinguish between those two possibilities. This is why the best description aligns with option describing those exact meanings. Other descriptions mix up what the responses indicate—for example, implying the port opens only when there’s a reset or mischaracterizing host reachability—so they don’t fit the standard definitions.