Quick Answer
The answer is Unfiltered, along with Open and Filtered, as the three valid Nmap port states from the given options. Unfiltered is correct because Nmap assigns this state when a port is accessible but Nmap cannot determine whether it is open or closed, typically occurring during an ACK scan where the target responds with an RST packet but no SYN/ACK or ICMP unreachable message is received. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of Nmap’s port state classification beyond the basic open and closed states, often appearing in questions about scan types like -sA or -sT. A common trap is confusing Unfiltered with Filtered—remember that Filtered means a firewall or filter is blocking the probe, while Unfiltered means the probe reached the port but the state remains ambiguous. For a quick memory tip, think “Unfiltered = unanswered, but reachable; Filtered = firewall blocked the path.”
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 THREE of the following are valid Nmap port states?
Open
An open port has a service listening.
Filtered
A filtered port is blocked by a firewall or filter.
Blocked
Why wrong: Blocked is not an Nmap port state; Nmap uses 'filtered' instead.
Stealth
Why wrong: Stealth is a scan technique, not a port state.
Unfiltered
Unfiltered means the port is accessible but Nmap cannot determine if it's open or closed (e.g., ACK scan).
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Open
Option A is correct because Nmap classifies a port as 'open' when it receives a SYN/ACK response to a SYN probe, indicating that an application is actively listening on that port. This is the standard TCP three-way handshake behavior defined in RFC 793, and it is the most fundamental port state in Nmap's scanning logic.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Open
Why this is correct
An open port has a service listening.
Related concept
Read the scenario before looking for a memorised answer.
Filtered
Why this is correct
A filtered port is blocked by a firewall or filter.
Related concept
Read the scenario before looking for a memorised answer.
Blocked
Why it's wrong here
Blocked is not an Nmap port state; Nmap uses 'filtered' instead.
Stealth
Why it's wrong here
Stealth is a scan technique, not a port state.
Unfiltered
Why this is correct
Unfiltered means the port is accessible but Nmap cannot determine if it's open or closed (e.g., ACK scan).
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates confuse Nmap's scanning techniques (like stealth scan) with port states, or assume 'blocked' is a valid state when it is actually a synonym for 'filtered' that Nmap does not officially use.
Detailed technical explanation
Nmap determines port states by analyzing responses to crafted packets: an open port responds with SYN/ACK, a closed port with RST, a filtered port gives no response or an ICMP unreachable (type 3, code 13), and an unfiltered port responds to ACK probes but its open/closed status is unknown. In real-world scenarios, a stateful firewall may return RST for filtered ports, causing Nmap to misclassify them as closed, which is why the 'open|filtered' state exists when no response is received.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Footprinting, Reconnaissance and Scanning — This question tests Footprinting, Reconnaissance and Scanning — Read the scenario before looking for a memorised answer..
The correct answer is: Open — Option A is correct because Nmap classifies a port as 'open' when it receives a SYN/ACK response to a SYN probe, indicating that an application is actively listening on that port. This is the standard TCP three-way handshake behavior defined in RFC 793, and it is the most fundamental port state in Nmap's scanning logic.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
About these practice questions
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Same concept, more angles
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. Which TWO of the following are valid port states that Nmap can report? (Select 2)
easyWhy C: Nmap reports 'Open' when a port responds to a probe (e.g., SYN, ACK, or connect scan) with a positive acknowledgment, indicating a service is actively listening. This is one of the six fundamental port states defined in Nmap's output, directly derived from the TCP/IP protocol behavior during the scan.
Last reviewed: Jun 24, 2026
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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