The answer is that the port forwarding rule maps the public IP to the DMZ server, not the internal server, which is why internal users cannot reach the internal web server via the public IP. This occurs because the port forwarding configuration explicitly redirects external HTTP traffic (port 80) to the DMZ web server at 10.0.0.10, leaving the internal server at 192.168.1.10 completely unaddressed by any rule; without a corresponding destination NAT entry for the internal server, traffic from inside the network destined to 203.0.113.1:80 is never redirected to 192.168.1.10. On the Fortinet NSE 4 exam, this scenario tests your understanding that hairpin NAT is only needed when a port forwarding rule already exists for the target server—here, the missing rule is the root cause, not a missing hairpin configuration. A common trap is assuming internal access fails due to NAT reflection issues, but the real failure is the absence of a port forwarding rule for the internal server. Memory tip: if the server isn’t in the rule, the traffic won’t flow—check the destination before blaming hairpin.
NSE4 System and Network Administration Practice Question
This NSE4 practice question tests your understanding of system and network administration. 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.
A medium-sized enterprise has a FortiGate 100F in NAT/Route mode with three interfaces: port1 (WAN, 203.0.113.1/24, gateway 203.0.113.254), port2 (internal, 192.168.1.1/24), and port3 (DMZ, 10.0.0.1/24). The internal network hosts a web server at 192.168.1.10 and a mail server at 192.168.1.20. The DMZ hosts a public web server at 10.0.0.10 and a public DNS server at 10.0.0.20. The company has a single public IP 203.0.113.1. The administrator has configured the following:
- Port forwarding: external HTTP to DMZ web server (10.0.0.10:80) and external DNS to DMZ DNS server (10.0.0.20:53).
- Outbound NAT (IP Pool) for internal users to 203.0.113.1.
- Firewall policies allowing internal to external, DMZ to external, and external to DMZ (for forwarded services).
Users report that they can access the Internet but cannot reach the internal web server (192.168.1.10) via its public IP (203.0.113.1:80). The DMZ web server is accessible from the Internet. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The port forwarding rule maps the public IP to the DMZ server, not the internal server
The port forwarding rule explicitly maps external HTTP (port 80) to the DMZ web server at 10.0.0.10. Since the internal web server at 192.168.1.10 is not referenced in any port forwarding rule, traffic destined to the public IP 203.0.113.1:80 from the internal network will not be redirected to 192.168.1.10. This is the root cause of the issue, not a missing hairpin NAT or misconfigured outbound NAT.
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.
✗
The firewall policy from internal to DMZ is blocking traffic
Why it's wrong here
The policy allows internal to DMZ, so this is not the cause.
✗
Hairpin NAT is not enabled on the FortiGate
Why it's wrong here
Even if hairpin NAT were enabled, the destination would still be the DMZ server, not the internal server.
✓
The port forwarding rule maps the public IP to the DMZ server, not the internal server
Why this is correct
The port forwarding is set to DMZ server (10.0.0.10), so internal users cannot reach the internal server via the public IP.
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 IP Pool for outbound NAT is misconfigured
Why it's wrong here
The IP Pool works for outbound traffic; internal users can browse the Internet.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume hairpin NAT is the universal fix for internal access to public IPs, but they overlook that the port forwarding rule must first exist for the target internal server; without that rule, hairpin NAT has no effect.
Detailed technical explanation
How to think about this question
In FortiGate, port forwarding (VIP) rules define how incoming traffic on a specific public IP and port is translated to a private IP and port. If no VIP exists for the internal web server, traffic hitting the public IP on port 80 will be forwarded only to the DMZ server as configured. Hairpin NAT (set nat-hairpin enable on the VIP) would allow internal clients to reach the same VIP from inside the network, but it does not create a new mapping; it only enables the existing VIP to work for internal source IPs. Without a VIP for 192.168.1.10, the traffic is simply dropped or forwarded to the default route.
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.
System and Network Administration — This question tests System and Network Administration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The port forwarding rule maps the public IP to the DMZ server, not the internal server — The port forwarding rule explicitly maps external HTTP (port 80) to the DMZ web server at 10.0.0.10. Since the internal web server at 192.168.1.10 is not referenced in any port forwarding rule, traffic destined to the public IP 203.0.113.1:80 from the internal network will not be redirected to 192.168.1.10. This is the root cause of the issue, not a missing hairpin NAT or misconfigured outbound NAT.
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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Question Discussion
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