CEH Scanning Networks and Enumeration Practice Question
This CEH practice question tests your understanding of scanning networks and enumeration. 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.
During a penetration test, you discover that an internal web server responds to ICMP echo requests but does not respond to TCP SYN scans on port 80. However, when you browse to the server's IP using a browser, the web page loads successfully. What is the most likely reason for this behavior?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A stateful firewall is blocking inbound SYN packets to port 80 but allowing responses to outbound connections.
A stateful firewall tracks the state of network connections. When you browse to the server, your browser initiates an outbound TCP connection, and the firewall allows the return SYN-ACK packets as part of the established session. However, a standalone TCP SYN scan sends unsolicited SYN packets to port 80, which the firewall sees as a new inbound connection attempt and blocks, preventing the server from responding. The server's ICMP echo reply is allowed because ICMP is stateless and not typically filtered by the same rules.
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.
✓
A stateful firewall is blocking inbound SYN packets to port 80 but allowing responses to outbound connections.
Why this is correct
Stateful firewalls track connection states; they may block unsolicited SYN but allow replies.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The web server is running on a non-standard port that you did not scan.
Why it's wrong here
The scan targeted port 80 and got no response, but browsing works; so the server is reachable on port 80 via a different mechanism.
✗
The server's TCP/IP stack is misconfigured and does not respond to SYN scans.
Why it's wrong here
Misconfiguration would likely affect all TCP connections, not just scans.
✗
A web application firewall is blocking the SYN scan traffic.
Why it's wrong here
WAFs operate at layer 7, not blocking TCP handshake packets.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a server that responds to ICMP and serves web pages must be fully reachable on all ports, overlooking how stateful firewalls differentiate between unsolicited inbound SYN packets and responses to outbound connections.
Detailed technical explanation
How to think about this question
Stateful firewalls maintain a connection table (e.g., using the TCP SYN, SYN-ACK, ACK handshake to establish state). When a SYN scan sends a packet without prior outbound traffic, the firewall sees it as an unsolicited inbound connection and drops it. In contrast, a browser's outbound SYN triggers a state entry, allowing the return SYN-ACK. This behavior is why Nmap's '-sS' scan often fails through stateful firewalls, while a full connect scan ('-sT') may succeed if the firewall permits outbound-initiated traffic.
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.
Scanning Networks and Enumeration — This question tests Scanning Networks and Enumeration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A stateful firewall is blocking inbound SYN packets to port 80 but allowing responses to outbound connections. — A stateful firewall tracks the state of network connections. When you browse to the server, your browser initiates an outbound TCP connection, and the firewall allows the return SYN-ACK packets as part of the established session. However, a standalone TCP SYN scan sends unsolicited SYN packets to port 80, which the firewall sees as a new inbound connection attempt and blocks, preventing the server from responding. The server's ICMP echo reply is allowed because ICMP is stateless and not typically filtered by the same rules.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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