Quick Answer
The answer is the -f flag. This Nmap option fragments transmitted packets into smaller pieces, typically 8-byte fragments, which allows an attacker to evade intrusion detection systems by splitting the malicious payload across multiple IP fragments. Because many IDS solutions inspect only complete, reassembled packets or lack deep packet inspection for fragmented traffic, the fragmented data can slip past signature-based detection. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of network evasion techniques and how attackers bypass perimeter defenses; a common trap is confusing -f with the --mtu flag, which sets a custom fragment size rather than using the default 8-byte fragmentation. To remember it, think “f for fragment” or “f for fool the IDS.”
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.
Which of the following Nmap flags would an attacker use to evade IDS by sending fragmented IP packets?
-D
Why wrong: -D is for decoy scans, not fragmentation.
-g
Why wrong: -g specifies a source port, not fragmentation.
-sS
Why wrong: -sS is for SYN scan, not fragmentation.
-f
-f causes packet fragmentation to evade detection.
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 flag in Nmap instructs the tool to fragment the transmitted packets into smaller pieces (typically 8-byte fragments). This fragmentation is used to evade Intrusion Detection Systems (IDS) that rely on signature matching against complete, unfragmented packets, as the IDS may not reassemble the fragments before inspection or may fail to detect the malicious payload when spread across multiple 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.
-D
Why it's wrong here
-D is for decoy scans, not fragmentation.
-g
Why it's wrong here
-g specifies a source port, not fragmentation.
-sS
Why it's wrong here
-sS is for SYN scan, not fragmentation.
-f
Why this is correct
-f causes packet fragmentation to evade detection.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often confuse the -f flag with the -D (decoy) flag, thinking both are used for evasion, but -f specifically targets packet fragmentation to bypass IDS, while -D hides the source IP.
Detailed technical explanation
When Nmap uses -f, it sets the IP header's 'More Fragments' (MF) bit and splits the TCP/UDP payload into fragments of 8 bytes or a multiple thereof (controlled by the --mtu option). Some IDS/IPS devices only reassemble fragments in kernel space or have a limited fragment reassembly timeout, allowing an attacker to bypass detection by sending fragments slowly or out of order. In real-world scenarios, combining -f with --data-length can further obscure payloads by padding packets to avoid trivial pattern matching.
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: -f — The -f flag in Nmap instructs the tool to fragment the transmitted packets into smaller pieces (typically 8-byte fragments). This fragmentation is used to evade Intrusion Detection Systems (IDS) that rely on signature matching against complete, unfragmented packets, as the IDS may not reassemble the fragments before inspection or may fail to detect the malicious payload when spread across multiple 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
This CEH practice question is part of Courseiva's free EC-Council 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 CEH exam.
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