The answer is that Nmap reports a port as 'filtered' when a firewall is either dropping probe packets without any response or responding with ICMP unreachable errors. This occurs because the probe packets are being intercepted by a filtering device, preventing Nmap from definitively determining whether the port is open or closed. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of how firewalls and ACLs manipulate network traffic to obscure port states, often appearing in questions about evasion techniques or scan interpretation. A common trap is confusing 'filtered' with 'closed'—remember that 'closed' means the host actively sends a reset (RST) packet, while 'filtered' means no response or an administrative prohibition. For a quick memory tip, think of the word "filter" as a coffee filter: it blocks the grounds (probes) from passing through, leaving you uncertain what is on the other side.
CEH Footprinting, Reconnaissance and Scanning Practice Question
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. 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.
Which TWO of the following describe the state of a port when Nmap reports it as 'filtered'? (Select 2)
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 dropping the probe packets
Nmap reports a port as 'filtered' when it cannot determine whether the port is open or closed because probe packets are being dropped or blocked. Option C is correct because a firewall dropping probe packets (e.g., via a DROP rule in iptables or an ACL) prevents Nmap from receiving any response, leading to a 'filtered' state. Option D is correct because a firewall responding with ICMP unreachable errors (e.g., ICMP Type 3 Code 13, 'Communication Administratively Prohibited') also causes Nmap to classify the port as 'filtered', as the response indicates a filtering device is interfering.
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 sends a RST packet
Why it's wrong here
A RST response indicates closed/unfiltered state.
✗
The port is open and a service is listening
Why it's wrong here
An open port typically responds appropriately, leading to 'open' state.
✓
A firewall is dropping the probe packets
Why this is correct
If a firewall silently drops packets, Nmap sees no response and marks the port filtered.
Related concept
Read the scenario before looking for a memorised answer.
✓
A firewall is responding with ICMP unreachable errors
Why this is correct
If a firewall sends back ICMP unreachable, Nmap may mark the port as filtered because it cannot confirm open/closed.
Related concept
Read the scenario before looking for a memorised answer.
✗
The port is open but no service is responding
Why it's wrong here
If no response, Nmap may mark filtered or open|filtered, but not simply open.
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 filter is interfering with the probe, rather than the port's actual listening state.
Detailed technical explanation
How to think about this question
Nmap's 'filtered' state arises when probe packets are silently dropped (no response) or when ICMP unreachable messages (Type 3, Codes 1, 2, 3, 9, 10, or 13) are received, indicating a firewall or packet filter is blocking access. In real-world scenarios, stateful firewalls may drop packets without any ICMP response, while ACLs on routers often generate ICMP unreachable (Code 13) for denied traffic. Nmap uses the absence of a response or specific ICMP error types to distinguish 'filtered' from 'closed' or 'open' states.
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 dropping the probe packets — Nmap reports a port as 'filtered' when it cannot determine whether the port is open or closed because probe packets are being dropped or blocked. Option C is correct because a firewall dropping probe packets (e.g., via a DROP rule in iptables or an ACL) prevents Nmap from receiving any response, leading to a 'filtered' state. Option D is correct because a firewall responding with ICMP unreachable errors (e.g., ICMP Type 3 Code 13, 'Communication Administratively Prohibited') also causes Nmap to classify the port as 'filtered', as the response indicates a filtering device is interfering.
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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Question Discussion
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