Valid Nmap Port States: Open, Filtered, Unfiltered, and Others
Which THREE of the following are valid Nmap port states?
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.”
⚠ Common exam trap
A common mix-up: 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.
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
✓
Open
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.
Answer analysis
Option-by-option breakdown
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.
- ✓
Filtered
Why this is correct
A filtered port is blocked by a firewall or filter.
- ✗
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).
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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. Which TWO of the following are valid port states that Nmap can report? (Select 2)
easy- A.Unknown
- B.Secured
- ✓ C.Open
- ✓ D.Filtered
- E.Blocked
Why 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. It reports 'Filtered' when packet filtering (e.g., a firewall) prevents Nmap's probes from reaching the port, making it impossible to determine if the port is open or closed. These are two of the six fundamental port states defined in Nmap's output, directly derived from TCP/IP protocol behavior during the scan.
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.