Understanding Nmap Filtered Port Status
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"?
Quick Answer
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.”
⚠ Common exam trap
It's easy for candidates to confuse 'filtered' with 'closed', but 'closed' requires an RST response, while 'filtered' indicates no response or an ICMP block, typically due to a firewall.
Answer choices
Why each option matters
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
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.
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.
- ✗
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.
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Same concept, more angles
1 more way this is tested on CEH
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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.