Quick Answer
The answer is the Fragment scan (-f) and the Idle scan (-sI). The Fragment scan is correct because it splits TCP headers into multiple small IP fragments, making it harder for firewalls and IDS systems to reassemble and inspect the full packet; this evasion technique exploits the fact that many filtering devices only check the first fragment and pass subsequent fragments without scrutiny, allowing the scan to bypass detection. The Idle scan, while not directly using fragmentation, also evades detection by bouncing the scan off a zombie host, masking the attacker’s IP. On the Certified Ethical Hacker CEH exam, this tests your understanding of network evasion tactics, often appearing in questions about bypassing packet inspection. A common trap is confusing the Fragment scan with the TCP SYN scan (-sS), which does not fragment packets. Memory tip: think “Frag for Fragments, Idle for Invisible” to recall which two scans hide your traffic from firewalls.
This CEH practice question tests your understanding of scanning networks and enumeration. 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 TWO of the following Nmap scan types are typically used to evade firewalls and IDS systems by sending fragmented packets?
Fragment scan (-f)
Correct: Fragments packets to bypass firewalls.
SYN scan (-sS)
Why wrong: SYN scan is not specifically evasive; it is the default scan.
ACK scan (-sA)
Why wrong: ACK scan is used for firewall rule mapping, not evasion.
Idle scan (-sI)
Correct: Uses a zombie host to spoof IP, evading detection.
Xmas scan (-sX)
Why wrong: Xmas scan is detectable and not evasive.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Fragment scan (-f)
The Fragment scan (-f) is correct because it splits TCP headers into multiple small IP fragments, making it harder for firewalls and IDS systems to reassemble and inspect the full packet. This evasion technique exploits the fact that many filtering devices only check the first fragment and pass subsequent fragments without scrutiny, allowing the scan to bypass detection.
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.
Fragment scan (-f)
Why this is correct
Correct: Fragments packets to bypass firewalls.
Related concept
Read the scenario before looking for a memorised answer.
SYN scan (-sS)
Why it's wrong here
SYN scan is not specifically evasive; it is the default scan.
ACK scan (-sA)
Why it's wrong here
ACK scan is used for firewall rule mapping, not evasion.
Idle scan (-sI)
Why this is correct
Correct: Uses a zombie host to spoof IP, evading detection.
Related concept
Read the scenario before looking for a memorised answer.
Xmas scan (-sX)
Why it's wrong here
Xmas scan is detectable and not evasive.
Common exam traps
The trap here is that candidates often confuse the Fragment scan (-f) with the Idle scan (-sI) because both are evasion techniques, but the Idle scan uses a zombie host to spoof the source IP, not fragmentation, to evade detection.
Detailed technical explanation
The -f option in Nmap splits the TCP header (typically 20 bytes) into 8-byte fragments, forcing the target to reassemble them. Many older or misconfigured firewalls and IDS systems only inspect the first fragment for protocol anomalies, allowing subsequent fragments to pass unchecked. In real-world scenarios, this technique is less effective against modern stateful inspection firewalls that perform full reassembly, but it remains useful against simple packet-filtering firewalls or legacy IDS.
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
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FAQ
Scanning Networks and Enumeration — This question tests Scanning Networks and Enumeration — Read the scenario before looking for a memorised answer..
The correct answer is: Fragment scan (-f) — The Fragment scan (-f) is correct because it splits TCP headers into multiple small IP fragments, making it harder for firewalls and IDS systems to reassemble and inspect the full packet. This evasion technique exploits the fact that many filtering devices only check the first fragment and pass subsequent fragments without scrutiny, allowing the scan to bypass detection.
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 11, 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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