The answer is that the destination interface must be set to the internal interface, not wan1. This is because a Virtual IP (VIP) performs destination network address translation (DNAT) for inbound traffic, and the firewall policy must define the path the traffic takes after translation: the source interface is where traffic enters (wan1), and the destination interface is where it must exit to reach the internal server (e.g., internal or lan). On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of policy routing for VIPs—a common misconfiguration trap where both interfaces are set to wan1, which prevents the firewall from forwarding the decapsulated traffic to the internal network. The key insight is that the VIP itself does not change the interface; the policy controls the flow. A simple memory tip: “Inbound DNAT needs a turn—source is the door in, destination is the door out.”
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 Virtual IP (VIP) to map public IP 203.0.113.10 to internal server 10.0.1.10 on port 443. The firewall policy uses the VIP as the destination address. External users report they cannot connect. The administrator checks the policy and sees the destination interface is 'wan1' and source interface is 'wan1'. What is the most likely issue?
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 destination interface should be the internal interface, not wan1
For inbound DNAT, the firewall policy must have the incoming interface as source interface and internal interface as destination interface. If both are set to wan1, the traffic is not forwarded to the internal network.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 destination interface should be the internal interface, not wan1
Why this is correct
After DNAT, the packet is destined to internal server; the outgoing interface should be internal.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The policy needs NAT enabled
Why it's wrong here
VIP already provides DNAT; policy NAT is for source NAT.
✗
The source interface should be the internal interface
Why it's wrong here
Source interface should be wan1 (incoming).
✗
The VIP is not associated with the policy
Why it's wrong here
The VIP is used as destination address, so it is associated.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The destination interface should be the internal interface, not wan1 — For inbound DNAT, the firewall policy must have the incoming interface as source interface and internal interface as destination interface. If both are set to wan1, the traffic is not forwarded to the internal network.
What should I do if I get this NSE4 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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