The answer is that the zombie host is not truly idle, causing false results. An Nmap idle scan uses the -sI flag to spoof the target into sending packets to a zombie host with a predictable IP ID sequence, allowing the attacker to infer open ports by observing changes in that sequence. If the zombie is actively communicating with other hosts during the scan, its IP ID values increment unpredictably, corrupting the side-channel analysis and leading Nmap to report all ports as filtered or closed. On the CEH exam, this tests your understanding of stealth scanning techniques and the critical assumption of zombie idleness—a common trap is assuming the scan itself failed rather than recognizing the zombie’s activity. Remember the mnemonic: “Idle scan needs an idle zombie; a busy zombie gives a busy result.”
CEH Footprinting, Reconnaissance and Scanning Practice Question
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. 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.
A security analyst runs the Nmap command: nmap -sI 192.168.1.50 -p 80 10.0.0.1. The scan completes, but the target shows no open ports. What is the MOST likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Why wrong: If the port were wrong, the scan would still complete; the issue is with the zombie's idle state.
B
The target has a firewall blocking the decoy packets
Why wrong: In an idle scan, the target sees packets from the zombie, not the attacker; a firewall would likely drop them, but the result would still indicate filtered.
C
The target host is offline
Why wrong: The scan used a zombie; the target may be online but the result is unreliable due to zombie activity.
D
The zombie host is not truly idle, causing false results
Idle scan relies on the zombie's IP ID being predictable; if the zombie is active, the IP ID increments unpredictably, leading to false negatives.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The zombie host is not truly idle, causing false results
The -sI flag in Nmap performs an idle scan, which relies on a zombie host (192.168.1.50) with a globally predictable IP ID sequence to probe the target. If the zombie host is not truly idle—meaning it is sending or receiving other traffic during the scan—its IP ID values will increment unpredictably, corrupting the side-channel analysis and causing Nmap to report all ports as filtered or closed. This is the most likely reason for the false 'no open ports' result.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 analyst used the wrong port number
Why it's wrong here
If the port were wrong, the scan would still complete; the issue is with the zombie's idle state.
✗
The target has a firewall blocking the decoy packets
Why it's wrong here
In an idle scan, the target sees packets from the zombie, not the attacker; a firewall would likely drop them, but the result would still indicate filtered.
✗
The target host is offline
Why it's wrong here
The scan used a zombie; the target may be online but the result is unreliable due to zombie activity.
✓
The zombie host is not truly idle, causing false results
Why this is correct
Idle scan relies on the zombie's IP ID being predictable; if the zombie is active, the IP ID increments unpredictably, leading to false negatives.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a firewall or offline target is the cause, but the idle scan's success hinges entirely on the zombie's idle state, not on target-side filtering or host availability.
Detailed technical explanation
How to think about this question
Idle scan (RFC 793-based) works by sending a SYN/ACK to the zombie to obtain its initial IP ID, then spoofing a SYN packet from the zombie to the target; if the target port is open, it sends a SYN/ACK back to the zombie, causing the zombie's IP ID to increment by 2 (or more if other traffic occurs). Nmap then checks the zombie's final IP ID; an increment of 2 indicates the port is open, while no increment (or an unexpected value) suggests the port is closed/filtered or the zombie was not idle. Real-world scenarios where this fails include zombie hosts behind NAT, using randomized IP IDs, or being actively used during the scan.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CEH question in full detail.
Footprinting, Reconnaissance and Scanning — This question tests Footprinting, Reconnaissance and Scanning — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The zombie host is not truly idle, causing false results — The -sI flag in Nmap performs an idle scan, which relies on a zombie host (192.168.1.50) with a globally predictable IP ID sequence to probe the target. If the zombie host is not truly idle—meaning it is sending or receiving other traffic during the scan—its IP ID values will increment unpredictably, corrupting the side-channel analysis and causing Nmap to report all ports as filtered or closed. This is the most likely reason for the false 'no open ports' result.
What should I do if I get this CEH question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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