Quick Answer
The correct answer is that the -f flag fragments the IP packets into 8-byte fragments. This works by splitting the original packet into smaller pieces, each carrying a minimal portion of the TCP header, which forces an Intrusion Detection System (IDS) to reassemble the fragments before it can analyze the scan pattern—a process many signature-based systems handle poorly, allowing the scan to slip through undetected. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of evasion techniques, often appearing in questions about bypassing firewalls or IDS; a common trap is confusing -f with the --mtu option, which allows custom fragment sizes rather than the fixed 8-byte default. To remember, think “f for fragment, eight for evade”—the -f flag always chops packets into 8-byte pieces to hide the scan’s intent.
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 wants to evade an IDS while scanning a target network. The tester uses the Nmap command: nmap -sS -f 10.10.10.1. What does the -f flag accomplish?
It increases the timing template to T5 (insane)
Why wrong: Timing templates are set with -T0 to -T5.
It uses an idle scan by bouncing off a zombie host
Why wrong: Idle scan uses -sI flag.
It sends packets with a spoofed source IP address
Why wrong: Spoofing is done with -D (decoy) or -S (source IP).
It fragments the IP packets into 8-byte fragments
-f fragments the packet, often into 8-byte fragments (or 24 for -ff).
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
It fragments the IP packets into 8-byte fragments
The -f flag in Nmap instructs the tool to fragment the IP packets into 8-byte fragments (or smaller, depending on the MTU). This is a common evasion technique used to bypass Intrusion Detection Systems (IDS) and firewalls by splitting the TCP header across multiple packets, making it harder for signature-based detection to reassemble and match the scan pattern.
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.
It increases the timing template to T5 (insane)
Why it's wrong here
Timing templates are set with -T0 to -T5.
It uses an idle scan by bouncing off a zombie host
Why it's wrong here
Idle scan uses -sI flag.
It sends packets with a spoofed source IP address
Why it's wrong here
Spoofing is done with -D (decoy) or -S (source IP).
It fragments the IP packets into 8-byte fragments
Why this is correct
-f fragments the packet, often into 8-byte fragments (or 24 for -ff).
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 other Nmap options like -T (timing), -sI (idle scan), or -S (spoofing), because they all start with a single dash and are used for evasion or stealth, but each has a distinct function.
Detailed technical explanation
When -f is used, Nmap splits the IP datagram into 8-byte fragments, which can cause the IDS to miss the scan if it does not perform proper IP reassembly or if it is configured to ignore fragmented packets. This technique exploits the fact that many IDS/IPS systems only inspect the first fragment, allowing subsequent fragments to pass undetected. In practice, some modern firewalls and IDS systems will drop or reassemble fragmented packets, so this evasion is less effective against stateful inspection devices.
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: It fragments the IP packets into 8-byte fragments — The -f flag in Nmap instructs the tool to fragment the IP packets into 8-byte fragments (or smaller, depending on the MTU). This is a common evasion technique used to bypass Intrusion Detection Systems (IDS) and firewalls by splitting the TCP header across multiple packets, making it harder for signature-based detection to reassemble and match the scan pattern.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A security team observes repeated Nmap scans from an external IP address. The scans show fragmented IP packets. Which evasion technique is the attacker using?
mediumWhy A: The attacker is using fragmentation to evade detection by splitting the Nmap probe packets into smaller fragments. This technique bypasses simple packet filters and intrusion detection systems (IDS) that lack IP fragment reassembly capabilities, as the fragments are typically allowed through individually. Nmap's `-f` flag or `--mtu` option is used to send fragmented IP packets, making the scan less conspicuous.
Variation 2. Which THREE of the following are valid Nmap flags that can be used to evade detection by an IDS? (Select exactly 3.)
hardWhy B: Option B (-D) is correct because the decoy scan flag allows you to spoof multiple source IP addresses, making it difficult for an IDS to distinguish the real scanning host from the decoys. This technique floods the target with scan traffic from many IPs, obscuring the true origin and evading detection.
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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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