The correct answer is that a firewall is likely blocking the probe packets. When Nmap reports a port as 'filtered', it means the SYN packets sent during the scan were dropped or elicited no response—no SYN/ACK to indicate an open port, and no RST to indicate a closed one. This silence typically results from a firewall, packet filter, or network-level ACL intercepting the traffic, preventing Nmap from determining the port’s true status. On the Certified Ethical Hacker CEH exam, this concept tests your ability to interpret scan results and distinguish between open, closed, and filtered states, a common pitfall being to assume a filtered port is simply closed. Remember the memory tip: “Filtered means firewalled—no reply, no deny, just silence in the sky.”
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.
A penetration tester runs 'nmap -sS -p 80 --script http-title 192.168.1.100' and receives output indicating port 80 is 'filtered'. What does the 'filtered' state imply?
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 likely blocking the probe packets
The 'filtered' state in Nmap indicates that the probe packets (SYN packets for a SYN scan) were dropped or did not elicit any response, typically due to a firewall or packet filter. Since no SYN/ACK or RST was received, Nmap cannot determine if the port is open or closed, so it marks it as 'filtered'. This is distinct from an 'open' state (SYN/ACK received) or 'closed' state (RST received).
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 a service is listening
Why it's wrong here
If it were open, Nmap would report 'open'.
✓
A firewall is likely blocking the probe packets
Why this is correct
Filtered indicates that probes are being dropped or blocked, often by a firewall.
Related concept
Read the scenario before looking for a memorised answer.
✗
The service is running but the script failed
Why it's wrong here
The script would have run if the port were open; filtered prevents script execution.
✗
The port is closed and no service is listening
Why it's wrong here
Closed ports return RST packets; filtered does not.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse 'filtered' with 'closed' or assume it means the service is running but unreachable, when in fact 'filtered' specifically indicates the probe was blocked by a filtering device.
Detailed technical explanation
How to think about this question
When Nmap sends a SYN packet to a port and receives no response (or an ICMP unreachable message like type 3 code 13), it classifies the port as 'filtered'. This behavior is defined in Nmap's port scanning logic and is common when a stateful firewall drops the probe without sending any reply. In real-world scenarios, a 'filtered' result often requires further testing with different scan types (e.g., -sA for ACK scan) to confirm firewall 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 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.
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 likely blocking the probe packets — The 'filtered' state in Nmap indicates that the probe packets (SYN packets for a SYN scan) were dropped or did not elicit any response, typically due to a firewall or packet filter. Since no SYN/ACK or RST was received, Nmap cannot determine if the port is open or closed, so it marks it as 'filtered'. This is distinct from an 'open' state (SYN/ACK received) or 'closed' state (RST received).
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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