This 200-201 practice question tests your understanding of network intrusion analysis. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Given a packet capture showing TCP packets with flags: first packet SYN, second packet SYN-ACK, third packet ACK, then a fourth packet with RST flag. What should the analyst suspect?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Port scan
Correct. The completed handshake followed by RST is characteristic of a connect scan.
Normal traffic
Why wrong: Incorrect. Normal traffic would continue with data, not an immediate RST.
SYN flood
Why wrong: Incorrect. SYN floods send many SYNs without completing the handshake.
Denial of service
Why wrong: Incorrect. DoS attacks typically flood with traffic, not this pattern.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Port scan
The three-way handshake (SYN, SYN-ACK, ACK) completes a TCP connection, but the immediate RST after the ACK indicates the client terminated the connection without sending any application data. This pattern is characteristic of a port scan (e.g., using nmap's connect scan), where the scanner verifies the port is open by completing the handshake and then immediately resets to avoid leaving the connection half-open.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Port scan
Why this is correct
Correct. The completed handshake followed by RST is characteristic of a connect scan.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Normal traffic
Why it's wrong here
Incorrect. Normal traffic would continue with data, not an immediate RST.
SYN flood
Why it's wrong here
Incorrect. SYN floods send many SYNs without completing the handshake.
Denial of service
Why it's wrong here
Incorrect. DoS attacks typically flood with traffic, not this pattern.
Common exam traps
Cisco often tests the distinction between a completed three-way handshake followed by a reset (port scan) versus an incomplete handshake (SYN flood) or sustained data transfer (normal traffic).
Detailed technical explanation
In TCP, the RST flag is used to abruptly terminate a connection. During a connect scan, the scanner sends a SYN, receives SYN-ACK, sends ACK (completing the handshake), then sends RST to close the connection. This allows the scanner to determine if a port is open without leaving a lingering connection in the target's state table. Tools like nmap use this technique because it is stealthier than a half-open scan but still detectable by inspecting the packet sequence.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Network Intrusion Analysis — This question tests Network Intrusion Analysis — Read the scenario before looking for a memorised answer..
The correct answer is: Port scan — The three-way handshake (SYN, SYN-ACK, ACK) completes a TCP connection, but the immediate RST after the ACK indicates the client terminated the connection without sending any application data. This pattern is characteristic of a port scan (e.g., using nmap's connect scan), where the scanner verifies the port is open by completing the handshake and then immediately resets to avoid leaving the connection half-open.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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