200-201 Network Intrusion Analysis Practice Question
During a SYN scan, an attacker sends a SYN packet to a closed port on a target. What response does the target typically send back?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
RST
In a SYN scan, a closed port responds with a RST packet to reject the connection attempt.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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ICMP Port Unreachable
Why it's wrong here
ICMP Port Unreachable is generated by UDP-based probes or filtered traffic, not by TCP SYN scans against closed ports, which receive RST. It is tempting because ICMP errors do signal unreachable services, but TCP's reset mechanism handles closed ports, making ICMP the wrong protocol layer here.
- ✓
RST
Why this is correct
A closed port has no listener, so the target's TCP stack replies to the SYN with a RST packet, immediately refusing the connection. An open port would answer SYN-ACK instead. This RST-versus-SYN-ACK difference is exactly what lets a SYN scan distinguish open from closed ports.
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ACK
Why it's wrong here
A closed port replies with RST, not ACK; ACK only appears within established TCP sessions or as part of the three-way handshake. It is tempting because ACK confirms receipt in normal TCP traffic, but a SYN to a closed port triggers the reset flag, so ACK is never the standalone response.
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SYN-ACK
Why it's wrong here
SYN-ACK is returned only by open ports completing the handshake's second step; a closed port sends RST instead. It is tempting because SYN-ACK is the expected reply during a successful connection attempt, but the question specifies a closed port, so the target refuses rather than accepts.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-201 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-201 exam.