In a TCP connect scan, what indicates an open port?

• A. No response from the target
• B. A connection is made and the port is labeled as open, and the connection is torn down with a RESET
• C. The port responds with a SYN-ACK
• D. The port times out

Answer: (B)

In a TCP connect scan, you rely on the operating system to complete a full TCP three‑way handshake on each target port. If the port is open, the handshake finishes and the connect call succeeds, so the port is marked open. After establishing the connection, the scanner closes it, and the teardown may be performed quickly using a reset (RST) to tear the connection down. This behavior—successful handshake followed by an immediate close—is how an open port is identified in a connect scan.

For contrast, no response or a timeout usually means the port is filtered, and a SYN-ACK appears in a SYN scan rather than a connect scan.