The correct answer is that a firewall is blocking the probe packets, and Nmap cannot determine if the port is open. When Nmap performs a SYN scan (-sS) and sends a SYN packet to a port, a filtered status results when the probe receives no response or an ICMP unreachable message, such as type 3 code 13, indicating a network filter or ACL is silently dropping the traffic. This concept is critical for the Certified Ethical Hacker CEH exam, as it tests your ability to interpret scan results and identify defensive mechanisms like firewalls or intrusion prevention systems. A common trap is confusing “filtered” with “closed”—remember, closed ports send a RST packet, while filtered ports give no reply or an ICMP error. For a quick memory tip, think “Filtered = Firewall in the way; no RST, no play.”
CEH Footprinting, Reconnaissance and Scanning Practice Question
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. Compare every option against the stated constraints before choosing — the best answer satisfies all requirements, not just the most obvious one. 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 executes the following command: nmap -sS -p 1-1000 --script banner 192.168.1.10. After the scan, the tester notices several filtered ports. Which of the following BEST explains why Nmap reports a port as "filtered"?
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
✓
A firewall is blocking the probe packets, and Nmap cannot determine if the port is open
Option B is correct because Nmap's SYN scan (-sS) sends a SYN packet to the target port. When the probe receives no response or an ICMP unreachable message (e.g., type 3 code 13), Nmap classifies the port as 'filtered'. This typically indicates a firewall, ACL, or network filter is dropping the packets, preventing Nmap from determining whether the port is open or closed.
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.
✗
The port is open, and the service is responding with a banner
Why it's wrong here
An open port would respond with SYN/ACK and the banner script would retrieve a banner.
✓
A firewall is blocking the probe packets, and Nmap cannot determine if the port is open
Why this is correct
Filtered indicates that no response was received, typically because a firewall dropped the packet.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The port is open and actively listening
Why it's wrong here
Open ports respond with SYN/ACK in SYN scan. Filtered ports do not respond at all.
✗
The port is closed and the target sent an RST packet
Why it's wrong here
Closed ports send RST packets, which Nmap reports as closed, not filtered.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'filtered' with 'closed', but 'closed' requires an RST response, while 'filtered' indicates no response or an ICMP block, typically due to a firewall.
Detailed technical explanation
How to think about this question
In a SYN scan, Nmap sends a TCP SYN packet to the target port. If no response is received (or an ICMP unreachable is returned), the port is marked 'filtered'. This behavior is defined in RFC 793 and is a key technique for firewall detection. In real-world engagements, a 'filtered' result often requires further testing with different scan types (e.g., -sA for ACK scan) to confirm the filtering device's rules.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
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: A firewall is blocking the probe packets, and Nmap cannot determine if the port is open — Option B is correct because Nmap's SYN scan (-sS) sends a SYN packet to the target port. When the probe receives no response or an ICMP unreachable message (e.g., type 3 code 13), Nmap classifies the port as 'filtered'. This typically indicates a firewall, ACL, or network filter is dropping the packets, preventing Nmap from determining whether the port is open or closed.
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.
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 →
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 analyst runs the command: nmap -sS -p 80,443,8080 --script http-headers scanme.nmap.org. The output shows that port 80 is filtered. What does 'filtered' mean in this context?
hard
A.The port is open, but the service is not responding
B.The port is open and actively listening
C.The port is closed, but the target is responding with RST packets
✓ D.A firewall or IDS is preventing the probe from reaching the port
Why D: In Nmap, a 'filtered' port status indicates that the port is being blocked by a firewall, IDS, or other network filtering device, preventing the probe from reaching the target service. The -sS (SYN stealth scan) sends a SYN packet; if no response is received or an ICMP unreachable (type 3, code 13) is returned, Nmap marks the port as filtered. This does not mean the port is open or closed—it means the scan could not determine the state due to filtering.
Last reviewed: Jun 24, 2026
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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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