- A
The source interface on the policy is set to the WAN interface instead of the internal interface
If the source interface is wrong, the traffic will not match this policy. The administrator may have a different policy (or the implicit allow) allowing the traffic, but the logs show the implicit deny is being hit because the correct policy is not matched.
- B
Central NAT is enabled and overriding the policy-based NAT
Why wrong: Central NAT does not affect policy matching; it only changes NAT behavior.
- C
The policy has a schedule applied that is currently inactive
Why wrong: If the schedule were inactive, the traffic would be denied, not allowed.
- D
The policy is placed after the implicit deny rule
Why wrong: The implicit deny is always last; policies cannot be placed after it.
Quick Answer
The answer is that the source interface on the policy is set to the WAN interface instead of the internal interface. This is the most likely cause because a FortiGate firewall policy is interface-specific; traffic arriving on the internal interface will only match a policy whose source interface is also set to internal. When the source interface is misconfigured to WAN, the traffic from the 10.0.0.0/8 network fails to match the intended rule, causing the FortiGate to fall through to the implicit deny policy (ID 0), which logs and drops the session. On the Fortinet NSE 4 exam, this scenario tests your understanding of policy matching order and interface binding—a common trap where candidates assume any policy with the correct source address will match regardless of interface. A reliable memory tip: “Match the mouth, not just the message”—the interface where traffic enters must match the policy’s source interface, or the implicit deny will catch it.
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.
A network administrator configures a firewall policy allowing HTTP traffic from the internal network (10.0.0.0/8) to the internet. After applying the policy, users report they can browse the web, but the FortiGate logs show that all sessions are using the 'implicit deny' policy ID 0. 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 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 source interface on the policy is set to the WAN interface instead of the internal interface
Option A is correct because if the source interface on the firewall policy is set to the WAN interface instead of the internal interface, traffic arriving from the internal network (10.0.0.0/8) on the internal interface will not match that policy. The FortiGate then falls through to the implicit deny policy (ID 0), which logs and drops the session. Since HTTP traffic is still reaching the internet, it suggests that another mechanism (such as a default route or NAT) is allowing the traffic, but the policy matching fails due to the interface mismatch.
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 source interface on the policy is set to the WAN interface instead of the internal interface
Why this is correct
If the source interface is wrong, the traffic will not match this policy. The administrator may have a different policy (or the implicit allow) allowing the traffic, but the logs show the implicit deny is being hit because the correct policy is not matched.
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.
- ✗
Central NAT is enabled and overriding the policy-based NAT
Why it's wrong here
Central NAT does not affect policy matching; it only changes NAT behavior.
- ✗
The policy has a schedule applied that is currently inactive
Why it's wrong here
If the schedule were inactive, the traffic would be denied, not allowed.
- ✗
The policy is placed after the implicit deny rule
Why it's wrong here
The implicit deny is always last; policies cannot be placed after it.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a policy with correct source/destination addresses will match regardless of the interface setting, but FortiOS strictly enforces interface matching, and a mismatch causes the policy to be ignored, leading to the implicit deny being hit.
Detailed technical explanation
How to think about this question
In FortiOS, firewall policies are matched based on source interface, destination interface, source address, destination address, schedule, and service. If the source interface is incorrect, the policy is skipped entirely, and the FortiGate continues evaluating subsequent policies until it hits the implicit deny (ID 0). The implicit deny logs all denied sessions with policy ID 0, which is a key indicator for troubleshooting policy misconfigurations. Real-world scenarios often involve misconfigured interface bindings when multiple VLANs or DMZ interfaces are present.
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.
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Firewall Policies and NAT — study guide chapter
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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 — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The source interface on the policy is set to the WAN interface instead of the internal interface — Option A is correct because if the source interface on the firewall policy is set to the WAN interface instead of the internal interface, traffic arriving from the internal network (10.0.0.0/8) on the internal interface will not match that policy. The FortiGate then falls through to the implicit deny policy (ID 0), which logs and drops the session. Since HTTP traffic is still reaching the internet, it suggests that another mechanism (such as a default route or NAT) is allowing the traffic, but the policy matching fails due to the interface mismatch.
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.
About these practice questions
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Last reviewed: Jun 24, 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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