Which scan detects when a port is open after completing the three-way TCP handshake?

Which scan detects when a port is open after completing the three-way TCP handshake?

• A. ICMP ECHO ping scans
• B. UDP Ping Scan
• C. IP protocol ping
• D. TCP Connect / Full Open Scan

Answer: (D)

Port states for TCP are confirmed by how the handshake is handled. A full TCP connect scan completes the three-way handshake (SYN, SYN-ACK, ACK), which establishes a real connection to the port. If the port is open, the handshake completes and the scanner sees an established connection; if closed, the target responds with a reset and no connection is formed. This direct verification of an open TCP port through a completed handshake is exactly what a TCP Connect / Full Open Scan does, making it the best answer for detecting an open port after the handshake. The other scans focus on host reachability (ICMP ECHO), UDP services (UDP Ping Scan), or different IP-level probing, and do not confirm a TCP open port via a completed handshake.

Port states for TCP are confirmed by how the handshake is handled. A full TCP connect scan completes the three-way handshake (SYN, SYN-ACK, ACK), which establishes a real connection to the port. If the port is open, the handshake completes and the scanner sees an established connection; if closed, the target responds with a reset and no connection is formed. This direct verification of an open TCP port through a completed handshake is exactly what a TCP Connect / Full Open Scan does, making it the best answer for detecting an open port after the handshake. The other scans focus on host reachability (ICMP ECHO), UDP services (UDP Ping Scan), or different IP-level probing, and do not confirm a TCP open port via a completed handshake.