Quick Answer
The answer is the -f option, which fragments the probe packets into smaller IP fragments to bypass filters that drop complete UDP datagrams. When a target drops unfragmented UDP packets, as suspected in this scenario, fragmenting the probes can evade simple packet filters or intrusion detection systems that only inspect the full packet header, allowing the fragments to reach the target and potentially trigger a response. On the Certified Ethical Hacker CEH exam, this tests your understanding of Nmap scan evasion techniques, specifically how fragmentation helps bypass stateless firewalls or IDS/IPS that lack deep packet inspection for reassembled fragments. A common trap is confusing -f with other evasion options like -D for decoys or -g for source port spoofing, but remember that fragmentation directly alters packet size, not origin or port. Memory tip: think of -f as “fragment” to “fool” the filter.
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 penetration tester runs the following Nmap command: nmap -sU -sS -p 53,161,162,500 10.0.0.1 and receives no responses for UDP scans but standard results for TCP. The tester suspects the target is dropping all UDP packets. Which Nmap option could help increase the likelihood of UDP responses by fragmenting the probe?
-f
-f fragments the probe packets, potentially helping to evade detection or filters that drop undifferentiated UDP traffic.
-T4
Why wrong: -T4 sets timing template to aggressive, but does not fragment packets.
--reason
Why wrong: --reason displays reasons for port states, not fragmentation.
-Pn
Why wrong: -Pn skips host discovery, but does not affect fragmentation.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
-f
The -f option fragments the probe packets into smaller IP fragments. When a target drops unfragmented UDP packets, fragmenting the probes can sometimes bypass simple packet filters or IDS/IPS that drop larger or complete UDP datagrams, increasing the chance that the target will process and respond to the fragments.
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.
-f
Why this is correct
-f fragments the probe packets, potentially helping to evade detection or filters that drop undifferentiated UDP traffic.
Related concept
Read the scenario before looking for a memorised answer.
-T4
Why it's wrong here
-T4 sets timing template to aggressive, but does not fragment packets.
--reason
Why it's wrong here
--reason displays reasons for port states, not fragmentation.
-Pn
Why it's wrong here
-Pn skips host discovery, but does not affect fragmentation.
Common exam traps
The trap here is that candidates often confuse -f (fragmentation) with -T4 (timing) or -Pn (no ping), assuming any option that makes the scan 'faster' or 'more aggressive' will also bypass packet drops, when in fact fragmentation is the specific technique to alter packet structure.
Detailed technical explanation
Nmap's -f option splits the UDP probe into 8-byte fragments (by default) at the IP layer. This can evade simple stateless firewalls that drop UDP packets based on size or pattern, as each fragment is a separate IP packet that must be reassembled by the target. In real-world scenarios, some network devices or hosts may respond to fragmented UDP probes when they would drop an equivalent unfragmented probe, especially if the target's UDP service expects small packets (e.g., DNS queries on port 53).
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the CEH exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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: -f — The -f option fragments the probe packets into smaller IP fragments. When a target drops unfragmented UDP packets, fragmenting the probes can sometimes bypass simple packet filters or IDS/IPS that drop larger or complete UDP datagrams, increasing the chance that the target will process and respond to the fragments.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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