- A
There is a Central SNAT rule with higher priority that does not match the traffic
Why wrong: If a Central SNAT rule exists but doesn't match, it doesn't affect policy-based NAT; the policy-based NAT would still apply.
- B
The firewall policy does not have the IP Pool selected in the NAT section
The policy must explicitly reference the IP Pool under 'NAT' -> 'Use IP Pool'.
- C
The IP Pool is configured on the wrong outgoing interface
If the IP Pool is not associated with the correct egress interface, it may not apply.
- D
The IP Pool uses one-to-one NAT instead of overload
Why wrong: One-to-one still translates the source IP; the symptom would be different, not the original IP showing.
- E
Another firewall policy above the current one matches the traffic and either denies it or does not use NAT
Since FortiGate uses first-match, a higher policy can intercept the traffic before reaching the intended policy.
Quick Answer
The answer is that another firewall policy above the current one matching the traffic and either denying it or not using NAT is one of three possible causes. This occurs because FortiGate processes firewall policies sequentially from top to bottom, so if a higher-priority policy matches the traffic first, the lower policy with the IP Pool NAT is never evaluated, leaving the original source IP untranslated. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of policy order and NAT configuration, often appearing as a troubleshooting trap where administrators overlook policy sequencing or forget to attach the IP Pool to the specific policy’s NAT settings. A common memory tip is “first match wins,” so always verify that the policy using the IP Pool is positioned above any other policies that could intercept the VLAN traffic, and confirm the pool is explicitly selected under the policy’s NAT column.
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 is troubleshooting why traffic from a specific VLAN (192.168.10.0/24) to the internet is not being NATed correctly. The firewall policy allows the traffic with NAT enabled and uses an IP Pool (overload) for the source translation. The IP Pool is configured with the address 203.0.113.10. However, the traffic still shows the original source IP. Which THREE of the following could cause this issue? (Choose three.)
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
The firewall policy does not have the IP Pool selected in the NAT section
Possible causes: The policy may not have the IP Pool selected in the NAT settings; the policy order might be incorrect if another policy matches first; the IP Pool might be assigned to the wrong interface or contain a different IP range; or the traffic might be matching a different policy that doesn't use NAT. Central SNAT rules could also override, but the question specifies policy-based NAT.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
There is a Central SNAT rule with higher priority that does not match the traffic
Why it's wrong here
If a Central SNAT rule exists but doesn't match, it doesn't affect policy-based NAT; the policy-based NAT would still apply.
- ✓
The firewall policy does not have the IP Pool selected in the NAT section
- ✓
The IP Pool is configured on the wrong outgoing interface
Why this is correct
If the IP Pool is not associated with the correct egress interface, it may not apply.
Related concept
Access ports place end devices into a single VLAN.
- ✗
The IP Pool uses one-to-one NAT instead of overload
Why it's wrong here
One-to-one still translates the source IP; the symptom would be different, not the original IP showing.
- ✓
Another firewall policy above the current one matches the traffic and either denies it or does not use NAT
Why this is correct
Since FortiGate uses first-match, a higher policy can intercept the traffic before reaching the intended policy.
Related concept
Access ports place end devices into a single VLAN.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Trap categories for this question
Command / output trap
One-to-one still translates the source IP; the symptom would be different, not the original IP showing.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related NSE4 questions on switching, trunking, and access-port configuration.
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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 — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The firewall policy does not have the IP Pool selected in the NAT section — Possible causes: The policy may not have the IP Pool selected in the NAT settings; the policy order might be incorrect if another policy matches first; the IP Pool might be assigned to the wrong interface or contain a different IP range; or the traffic might be matching a different policy that doesn't use NAT. Central SNAT rules could also override, but the question specifies policy-based NAT.
What should I do if I get this NSE4 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related NSE4 questions on switching, trunking, and access-port configuration.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 21, 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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