The answer is that the firewall policy has NAT enabled, which changes the source IP to the FortiGate’s internal interface IP. This happens because, by default, when a VIP is used as the destination in a policy, FortiGate automatically performs source NAT (SNAT) on the traffic unless you explicitly disable NAT on that policy. The VIP handles destination translation, but without disabling NAT, the firewall rewrites the source address to its own egress interface IP, masking the original client. On the Fortinet NSE 4 exam, this is a classic trap: candidates often assume VIPs preserve the source IP, but the default behavior is to NAT the source unless the policy has NAT set to "off." A common memory tip is "VIP maps the destination, but NAT hides the source"—so to keep the original client IP visible in server logs, you must disable NAT on the policy.
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 admin configures a VIP to map a public IP to an internal server. The firewall policy uses the VIP as the destination. External users can access the server, but the server's logs show the source IP as the FortiGate's internal interface IP instead of the original client IP. Why is this happening?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The firewall policy has NAT enabled, which changes the source IP to the FortiGate's egress interface IP
By default, when using NAT (including VIP), FortiGate performs source NAT (SNAT) for the traffic destined to the VIP unless specifically configured otherwise. To preserve the original source IP, the admin must disable NAT on the policy or use a policy with NAT disabled.
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 VIP is configured with port forwarding and the server is expecting a different port
Why it's wrong here
Port forwarding does not affect source IP preservation.
✗
The VIP is using a different public IP than expected
Why it's wrong here
The VIP mapping is correct since external users can access the server.
✓
The firewall policy has NAT enabled, which changes the source IP to the FortiGate's egress interface IP
Why this is correct
Correct. If NAT is enabled on the policy, FortiGate performs SNAT, hiding the original source.
Related concept
Static NAT maps one inside address to one outside address.
✗
The server's routing is misconfigured and traffic is returning via a different path
Why it's wrong here
Asymmetric routing might cause issues, but the logs would show the source IP as the client if NAT were disabled.
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.
Trap categories for this question
Command / output trap
Asymmetric routing might cause issues, but the logs would show the source IP as the client if NAT were disabled.
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 firewall policy has NAT enabled, which changes the source IP to the FortiGate's egress interface IP — By default, when using NAT (including VIP), FortiGate performs source NAT (SNAT) for the traffic destined to the VIP unless specifically configured otherwise. To preserve the original source IP, the admin must disable NAT on the policy or use a policy with NAT disabled.
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.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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