CEH Footprinting, Reconnaissance and Scanning Practice Question
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.
During a penetration test, you need to identify all live hosts on a target network without being detected by intrusion detection systems. Which Nmap flag would BEST achieve this?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
-sn (ping sweep)
The -sn flag (ping sweep) sends ICMP echo requests, TCP SYN to port 443, TCP ACK to port 80, and ICMP timestamp requests by default to determine if a host is alive. This is the best choice for stealthy host discovery because it does not complete a full TCP handshake or send application-layer probes, minimizing the chance of triggering IDS signatures that look for port scans or OS fingerprinting.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
-O (OS fingerprinting)
Why it's wrong here
OS fingerprinting requires active probes that can be detected by IDS.
✓
-sn (ping sweep)
Why this is correct
Ping sweep only checks host availability, generating minimal traffic and avoiding full port scans.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
-sS (SYN scan)
Why it's wrong here
SYN scan is stealthier than connect scan but still actively probes ports, increasing detection risk.
✗
-sV (version detection)
Why it's wrong here
Version detection connects to open ports and may trigger alerts.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse -sn (ping sweep) with -sS (SYN scan) because both involve TCP packets, but -sn is purely for host discovery without port scanning, whereas -sS is a port scanning technique that is much more intrusive and detectable.
Detailed technical explanation
How to think about this question
Nmap's -sn flag uses a multi-method discovery approach: by default on a local network, it sends an ARP request (which is non-IP and often not monitored by IDS), while on a remote network it sends ICMP echo requests, TCP SYN to port 443, TCP ACK to port 80, and ICMP timestamp requests. This combination increases the chance of detecting live hosts even when firewalls block ICMP, but the lack of full connection establishment keeps the scan relatively low-profile compared to SYN or version scans.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CEH question in full detail.
Footprinting, Reconnaissance and Scanning — This question tests Footprinting, Reconnaissance and Scanning — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: -sn (ping sweep) — The -sn flag (ping sweep) sends ICMP echo requests, TCP SYN to port 443, TCP ACK to port 80, and ICMP timestamp requests by default to determine if a host is alive. This is the best choice for stealthy host discovery because it does not complete a full TCP handshake or send application-layer probes, minimizing the chance of triggering IDS signatures that look for port scans or OS fingerprinting.
What should I do if I get this CEH question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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