The answer is that Central NAT is enabled and overrides the per-policy NAT setting. When Central NAT is active on a FortiGate, it takes precedence over any NAT configuration applied directly within a firewall policy, including policy-based NAT intended to translate the source IP to the interface IP. This means that even if the firewall policy has NAT enabled, the central NAT rules will govern the translation, often resulting in the original source IP being sent out instead of the interface IP. On the Fortinet NSE 4 Network Security Professional exam, this scenario tests your understanding of the NAT processing order and the critical distinction between policy-based NAT and Central NAT—a common trap is assuming that enabling NAT in the policy alone is sufficient. Remember the memory tip: "Central overrides per-policy; disable Central for per-policy to stick."
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 FortiGate has policy-based NAT enabled. The admin wants to translate the source IP of internal users to the interface IP for internet traffic. The firewall policy has NAT enabled. However, traffic from the internal network to the internet shows the original source IP instead of the interface IP. What is the MOST likely reason?
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.
Central NAT is enabled and overrides the per-policy NAT setting
With central NAT enabled, the policy's NAT flag is ignored; central NAT rules are used instead.
B
The destination is a VIP that disables NAT
Why wrong: VIPs are for destination NAT; they do not affect source NAT on the same policy unless configured to disable NAT, but that would be intentional.
C
The NGFW mode is set to profile-based
Why wrong: NGFW mode does not impact NAT functionality.
D
The policy is configured in proxy inspection mode
Why wrong: Proxy mode does not affect NAT operation.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Central NAT is enabled and overrides the per-policy NAT setting
Central NAT must be disabled for policy-based NAT to work. When central NAT is enabled, it overrides the per-policy NAT settings.
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.
✓
Central NAT is enabled and overrides the per-policy NAT setting
Why this is correct
With central NAT enabled, the policy's NAT flag is ignored; central NAT rules are used instead.
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 destination is a VIP that disables NAT
Why it's wrong here
VIPs are for destination NAT; they do not affect source NAT on the same policy unless configured to disable NAT, but that would be intentional.
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: Central NAT is enabled and overrides the per-policy NAT setting — Central NAT must be disabled for policy-based NAT to work. When central NAT is enabled, it overrides the per-policy NAT settings.
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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