The correct answer is that a firewall, router rule, or host-based firewall is blocking the probes. This is because when Nmap reports a port as 'filtered', it indicates that the scan probes—such as SYN packets—were dropped or blocked before reaching the target service, resulting in no response or an ICMP unreachable message, unlike an 'open' port which replies with a SYN/ACK. On the Certified Ethical Hacker CEH exam, this concept tests your ability to interpret scan results and distinguish between firewall interference and actual service availability; a common trap is confusing 'filtered' with 'closed', but remember that 'closed' returns a RST packet, while 'filtered' yields silence or an error. The key insight is that 'filtered' means Nmap cannot confirm whether the port is open or closed due to a blocking device. A useful memory tip is: "Filtered = Firewall fumbled the packet."
CEH Scanning Networks and Enumeration Practice Question
This CEH practice question tests your understanding of scanning networks and enumeration. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
Refer to the exhibit.
```
Starting Nmap 7.92 ( https://nmap.org ) at 2025-03-25 14:22 EDT
Nmap scan report for 10.10.1.45
Host is up (0.045s latency).
Not shown: 997 closed tcp ports (reset)
PORT STATE SERVICE
22/tcp open ssh
80/tcp filtered http
443/tcp open https
```
Refer to the exhibit. An Nmap scan shows that port 80 is 'filtered' while ports 22 and 443 are 'open'. What does the 'filtered' state indicate?
Refer to the exhibit.
```
Starting Nmap 7.92 ( https://nmap.org ) at 2025-03-25 14:22 EDT
Nmap scan report for 10.10.1.45
Host is up (0.045s latency).
Not shown: 997 closed tcp ports (reset)
PORT STATE SERVICE
22/tcp open ssh
80/tcp filtered http
443/tcp open https
```
A
The port is closed and the target sent a RST packet.
Why wrong: Closed ports send RST, which Nmap reports as 'closed'.
B
The port is open but the service is not responding to the scan.
Why wrong: If open, Nmap would detect a SYN/ACK response.
C
The port is open but Nmap cannot determine the service.
Why wrong: Nmap distinguishes filtered from unfiltered; unfiltered means open/closed but no version.
D
A firewall, router rule, or host-based firewall is blocking the probes.
Filtered indicates that probes are being dropped or blocked.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A firewall, router rule, or host-based firewall is blocking the probes.
When Nmap reports a port as 'filtered', it means that the scan probes (e.g., SYN packets) were dropped or blocked before reaching the target service, typically by a firewall, router ACL, or host-based firewall. Unlike 'open' (which receives a SYN/ACK) or 'closed' (which receives a RST), 'filtered' indicates no response or an ICMP unreachable message, so Nmap cannot confirm whether the port is actually 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 closed and the target sent a RST packet.
Why it's wrong here
Closed ports send RST, which Nmap reports as 'closed'.
✗
The port is open but the service is not responding to the scan.
Why it's wrong here
If open, Nmap would detect a SYN/ACK response.
✗
The port is open but Nmap cannot determine the service.
Why it's wrong here
Nmap distinguishes filtered from unfiltered; unfiltered means open/closed but no version.
✓
A firewall, router rule, or host-based firewall is blocking the probes.
Why this is correct
Filtered indicates that probes are being dropped or blocked.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'filtered' with 'closed' or 'open', not realizing that 'filtered' specifically indicates a firewall or ACL is interfering with the probe, not the state of the service itself.
Detailed technical explanation
How to think about this question
In a TCP SYN scan (-sS), Nmap sends a SYN packet to the target port. If a firewall silently drops the packet (no response), Nmap retransmits and eventually marks the port as 'filtered'. If an ICMP Type 3 Code 13 (administratively prohibited) is received, Nmap also marks it 'filtered'. This behavior is defined in the Nmap source code and RFC 1122; real-world scenarios include cloud security groups or iptables rules that drop inbound traffic to port 80 while allowing 22 and 443.
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.
Scanning Networks and Enumeration — This question tests Scanning Networks and Enumeration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A firewall, router rule, or host-based firewall is blocking the probes. — When Nmap reports a port as 'filtered', it means that the scan probes (e.g., SYN packets) were dropped or blocked before reaching the target service, typically by a firewall, router ACL, or host-based firewall. Unlike 'open' (which receives a SYN/ACK) or 'closed' (which receives a RST), 'filtered' indicates no response or an ICMP unreachable message, so Nmap cannot confirm whether the port is actually 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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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