Question 1hardmultiple choice
Review the full subnetting walkthrough →Question 904 of 1,000
Quick Answer
The answer is that NAT reflection (hairpin NAT) is not enabled. This is the most likely cause because when an internal user on the same subnet as the server sends a packet to the public VIP, the FortiGate performs destination NAT to the private IP but, without hairpin NAT, does not translate the source IP. The server then sees the source as the internal user’s private IP and replies directly to that address, bypassing the FortiGate and breaking the return path due to asymmetric routing. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of how VIPs handle traffic flows differently for internal versus external users; a common trap is assuming DNAT alone suffices for all users. Remember the key distinction: hairpin NAT ensures the source IP is also translated so the reply goes back through the FortiGate. A handy memory tip is “same interface, same subnet—hairpin must be set.”
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. 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.
An administrator configures a VIP for inbound HTTP traffic to an internal server (192.168.1.10:80). External users can reach the server via the VIP, but internal users on the same subnet as the server cannot access the server using its public IP. What is the most likely cause?
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 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
✓
NAT reflection (hairpin NAT) is not enabled
Option D is correct because internal users on the same subnet as the server (192.168.1.10) attempting to reach the server via its public VIP (e.g., 203.0.113.10) will send packets to the FortiGate, which performs destination NAT (DNAT) to the private IP. Without NAT reflection (also called hairpin NAT or NAT loopback), the FortiGate does not translate the source IP for traffic that enters and leaves the same interface, so the server sees the source as the internal user's private IP and replies directly, bypassing the FortiGate. This breaks the return path because the internal user's packet was destined for the VIP but the reply comes from the private IP, causing asymmetric routing and connection failure.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume internal users can reach the server via the public IP because the VIP works for external users, overlooking the fact that hairpin NAT is a separate feature required for traffic that enters and exits the same FortiGate interface.
Detailed technical explanation
How to think about this question
Hairpin NAT requires the FortiGate to perform source NAT (SNAT) on the internal user's source IP when the traffic is DNATed to the internal server, ensuring the server replies to the FortiGate (which then reverses the SNAT) rather than directly to the internal user. In FortiOS, this is enabled via the 'set nat-reflection enable' command under the VIP configuration or globally via 'config system settings' with 'set nat-hairpin enable'. Without it, the FortiGate only performs DNAT for traffic entering from the WAN interface, not for traffic that originates and terminates on the same subnet.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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Firewall Policies and NAT — study guide chapter
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FAQ
Questions learners often ask
What does this NSE4 question test?
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: NAT reflection (hairpin NAT) is not enabled — Option D is correct because internal users on the same subnet as the server (192.168.1.10) attempting to reach the server via its public VIP (e.g., 203.0.113.10) will send packets to the FortiGate, which performs destination NAT (DNAT) to the private IP. Without NAT reflection (also called hairpin NAT or NAT loopback), the FortiGate does not translate the source IP for traffic that enters and leaves the same interface, so the server sees the source as the internal user's private IP and replies directly, bypassing the FortiGate. This breaks the return path because the internal user's packet was destined for the VIP but the reply comes from the private IP, causing asymmetric routing and connection failure.
What should I do if I get this NSE4 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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Last reviewed: Jun 24, 2026
This NSE4 practice question is part of Courseiva's free Fortinet 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 NSE4 exam.
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