If an application is actively accepting TCP connections on a port, what is the likely port state?

• A. Open
• B. Closed
• C. Filtered
• D. Unknown

Answer: (A)

Open is the state you see when a TCP service is actively listening on a port and ready to accept connections. That means the application is bound to the port, in a listening state, and a client can complete the TCP handshake to establish a connection. If no service is listening, the port is closed and probes are typically rejected or reset. If a firewall or filter blocks the probes, the port appears as filtered because there’s no reliable response. If the scanner can’t determine the state from the responses, it shows unknown.