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.
During a vulnerability assessment, a security analyst receives an alert from the IDS that a scan with fragmented packets and spoofed source IPs is targeting the internal network. Which Nmap command MOST likely caused this alert?
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.
nmap -sS -O 192.168.1.1
Why wrong: SYN scan with OS detection; no fragmentation or decoys.
nmap -sV -p 80 192.168.1.1
Why wrong: Version detection on port 80; no fragmentation or decoys.
nmap -sU 192.168.1.1
Why wrong: UDP scan; no fragmentation or decoys.
nmap -f -D 10.0.0.1,10.0.0.2 192.168.1.1
-f fragments packets, -D adds decoy IPs. This matches the IDS alert description.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
nmap -f -D 10.0.0.1,10.0.0.2 192.168.1.1
Option D is correct because the `-f` flag fragments the packets into smaller IP fragments, and the `-D` flag performs a decoy scan by spoofing source IPs. This combination causes the IDS to detect fragmented packets with spoofed source addresses, matching the alert description.
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.
nmap -sS -O 192.168.1.1
Why it's wrong here
SYN scan with OS detection; no fragmentation or decoys.
nmap -sV -p 80 192.168.1.1
Why it's wrong here
Version detection on port 80; no fragmentation or decoys.
nmap -sU 192.168.1.1
Why it's wrong here
UDP scan; no fragmentation or decoys.
nmap -f -D 10.0.0.1,10.0.0.2 192.168.1.1
Common exam traps
The trap here is that candidates may confuse `-f` with other scan types like SYN or UDP scans, but the key is recognizing that fragmentation and spoofed source IPs are explicitly enabled by `-f` and `-D` respectively.
Detailed technical explanation
Fragmentation (`-f`) splits the TCP header into multiple IP fragments, often bypassing simple IDS signatures that expect complete headers. Decoy scans (`-D`) generate random source IPs in addition to the real one, making it difficult for the IDS to identify the true scanning host. In practice, combining these techniques can evade detection by legacy intrusion detection systems that lack proper IP reassembly or source IP correlation.
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
Footprinting, Reconnaissance and Scanning — This question tests Footprinting, Reconnaissance and Scanning — Read the scenario before looking for a memorised answer..
The correct answer is: nmap -f -D 10.0.0.1,10.0.0.2 192.168.1.1 — Option D is correct because the `-f` flag fragments the packets into smaller IP fragments, and the `-D` flag performs a decoy scan by spoofing source IPs. This combination causes the IDS to detect fragmented packets with spoofed source addresses, matching the alert description.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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