This CEH practice question tests your understanding of introduction to ethical hacking. 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 penetration test, an ethical hacker needs to evade an IDS that detects port scans based on the number of packets per second. Which technique would be most effective to avoid detection?
Use random source ports
Why wrong: Random ports don't affect packet rate.
Use a decoy scan
Why wrong: Decoy scans can still generate high packet rates.
Slow down the scan rate
Reducing packets per second avoids triggering rate-based IDS thresholds.
Use fragmented packets
Why wrong: Fragmentation evades signature detection, not rate-based 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
Slow down the scan rate
Option C is correct because slowing down the scan rate reduces the number of packets sent per second below the IDS threshold, allowing the scan to blend in with normal traffic. IDS systems like Snort use packet-per-second (pps) counters to detect port scans; by spacing out packets over a longer period, the scan avoids triggering these rate-based alerts.
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.
Use random source ports
Why it's wrong here
Random ports don't affect packet rate.
Use a decoy scan
Why it's wrong here
Decoy scans can still generate high packet rates.
Slow down the scan rate
Why this is correct
Reducing packets per second avoids triggering rate-based IDS thresholds.
Related concept
Read the scenario before looking for a memorised answer.
Use fragmented packets
Why it's wrong here
Fragmentation evades signature detection, not rate-based detection.
Common exam traps
EC-Council often tests the misconception that fragmentation alone evades IDS, but candidates must remember that rate-based detection counts packets regardless of fragmentation, so slowing the scan is the direct countermeasure.
Detailed technical explanation
Rate-based IDS detection relies on sliding window counters, often tracking packets per second over intervals like 1-5 seconds. Tools like Nmap's --scan-delay or -T0 (paranoid timing) can slow scans to as low as 0.5 packets per second, staying under typical thresholds (e.g., Snort's default portscan alert triggers at 5 pps). In real-world scenarios, attackers may also use random delays between probes to further mimic legitimate traffic patterns.
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
Introduction to Ethical Hacking — This question tests Introduction to Ethical Hacking — Read the scenario before looking for a memorised answer..
The correct answer is: Slow down the scan rate — Option C is correct because slowing down the scan rate reduces the number of packets sent per second below the IDS threshold, allowing the scan to blend in with normal traffic. IDS systems like Snort use packet-per-second (pps) counters to detect port scans; by spacing out packets over a longer period, the scan avoids triggering these rate-based alerts.
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 30, 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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