The answer is that the firewall policy for outbound traffic does not have NAT enabled. This is the most likely cause because in NAT/Route mode, a FortiGate must perform source NAT (masquerading) to translate the private 192.168.1.0/24 source IP to the public interface IP 10.0.0.1 before sending traffic to the internet; without this, packets retain a non-routable private source address, so the public server at 203.0.113.5 cannot send return traffic back. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of how NAT/Route mode internet access troubleshooting differs from transparent mode, and it is a common trap to assume routing alone is sufficient when a static default route exists. A helpful memory tip is “private source needs a public face”—if outbound traffic from a private subnet fails to reach a specific public server while general browsing works, always check that NAT is enabled on the policy.
NSE4 System and Network Administration Practice Question
This NSE4 practice question tests your understanding of system and network administration. 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 FortiGate configured in NAT/Route mode is connected to the internet via port1 with an IP 10.0.0.1/24. The internal network uses 192.168.1.0/24. Users can browse the internet but cannot reach a public server at 203.0.113.5. A static default route exists. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The firewall policy for outbound traffic does not have NAT enabled
Option C is correct because in NAT/Route mode, outbound traffic from a private IP (192.168.1.0/24) to a public internet destination requires source NAT (masquerading) to translate the private source IP to the interface IP (10.0.0.1). Without NAT enabled on the firewall policy, the FortiGate forwards the packet with the original private source IP, which is not routable on the public internet, so the return traffic cannot reach the internal host. The static default route exists and users can browse other sites, indicating routing is functional, but the specific public server is unreachable due to missing NAT.
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 static default route points to the wrong gateway
Why it's wrong here
If users can browse the internet, the default route is likely correct.
✗
The public server is down
Why it's wrong here
This is possible but not the most likely given the scenario.
✓
The firewall policy for outbound traffic does not have NAT enabled
Why this is correct
Without NAT, internal private IPs are sent untranslated and cannot reach the internet.
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.
✗
The DNS server is not configured
Why it's wrong here
DNS would affect name resolution, but the user is trying to reach a specific IP.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a working internet connection implies all outbound policies are correct, but FortiGate requires NAT to be explicitly enabled per policy, and missing NAT can cause selective unreachability while other traffic (e.g., to servers with public IPs that accept private source IPs) might still work.
Trap categories for this question
Scenario analysis trap
This is possible but not the most likely given the scenario.
Detailed technical explanation
How to think about this question
In NAT/Route mode, FortiGate performs destination-based routing and requires explicit NAT configuration in the firewall policy (using the 'set nat enable' command or GUI toggle) to translate private source IPs to the egress interface IP. Without NAT, the FortiGate sends packets with the original source IP (192.168.1.x), which ISPs drop because RFC 1918 addresses are not routable on the public internet. A common real-world scenario is when a single public server is unreachable while other sites work, often due to asymmetric routing or missing NAT for specific destinations, but here the root cause is the lack of source NAT on the policy.
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.
System and Network Administration — This question tests System and Network Administration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall policy for outbound traffic does not have NAT enabled — Option C is correct because in NAT/Route mode, outbound traffic from a private IP (192.168.1.0/24) to a public internet destination requires source NAT (masquerading) to translate the private source IP to the interface IP (10.0.0.1). Without NAT enabled on the firewall policy, the FortiGate forwards the packet with the original private source IP, which is not routable on the public internet, so the return traffic cannot reach the internal host. The static default route exists and users can browse other sites, indicating routing is functional, but the specific public server is unreachable due to missing NAT.
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.
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Question Discussion
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