200-201 Network Intrusion Analysis Practice Question
An analyst is analyzing a PCAP and sees multiple ICMP port unreachable responses from a target host when scanning UDP ports. What does this indicate about the scanned ports?
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
✓
The ports are closed.
When a UDP scan sends a packet to a closed port, the target responds with an ICMP port unreachable message.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The ports are closed.
Why this is correct
An ICMP port unreachable response means the target host received the UDP datagram but no application is listening on that port, which is the definitive indicator that the scanned port is closed rather than filtered.
- ✗
The scan is a SYN scan.
Why it's wrong here
ICMP port unreachable is a UDP-specific response, so it cannot indicate a SYN scan, which targets TCP and elicits SYN-ACK or RST. It is tempting because SYN scanning is the common stealth technique, and it would be the correct answer if the capture showed half-open TCP handshakes without completion.
- ✗
The ports are filtered by a firewall.
Why it's wrong here
A firewall silently dropping UDP probes produces no response at all, so filtered ports yield timeouts rather than ICMP port unreachable. It is tempting because firewalls commonly filter ports, and this would be correct if the PCAP showed only retransmitted probes with no replies from the target.
- ✗
The ports are open.
Why it's wrong here
ICMP port unreachable means the host received the UDP datagram but no application is listening, so the port is closed, not open. It is tempting because a reply arrived from the target, and this would be correct if the host had answered with a UDP payload or service banner.
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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.