The correct answer is policy-based NAT using an IP pool with type 'Overload' and the IP address 203.0.113.10. This configuration works because policy-based NAT allows you to define a specific source IP translation for traffic matching a particular policy, and the Overload IP pool type implements Port Address Translation (PAT), enabling multiple internal hosts using RFC 1918 addresses to share the single public IP 203.0.113.10 when accessing the external FTP server. On the Fortinet NSE 4 exam, this scenario tests your understanding of how to apply fixed source IP translation without breaking connectivity for multiple users—a common trap is choosing Central NAT or a Fixed Port Range pool, which would either not apply to the policy or waste addresses. Remember the key distinction: policy-based NAT with Overload is for many-to-one translation to a specific IP, while Fixed Port Range is for one-to-one mapping. A helpful memory tip is "Policy picks the pool, Overload shares the load."
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 administrator wants to ensure that traffic from the internal network to an external FTP server uses a specific source IP address (203.0.113.10). The internal network uses RFC 1918 addresses. Which NAT configuration should be used?
Policy-based NAT using an IP pool set to 'Fixed Port Range'
Why wrong: Fixed port range is a type of IP pool for port allocation, not for specifying a single source IP.
B
Virtual IP (VIP) mapping the internal server to 203.0.113.10
Why wrong: VIP is for inbound DNAT (destination NAT), not for outbound source translation.
C
Central SNAT with dynamic IP pool
Why wrong: Central SNAT can use IP pools, but the question implies policy-based; both could work, but policy-based is more direct for specific destination.
D
Policy-based NAT using an IP pool with type 'Overload' and the IP address 203.0.113.10
An IP pool with type 'Overload' (PAT) using a single IP will translate all matching sessions to that IP address. This meets the requirement.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Policy-based NAT using an IP pool with type 'Overload' and the IP address 203.0.113.10
Option D is correct because policy-based NAT with an IP pool type 'Overload' (PAT) allows multiple internal hosts to share the single public IP 203.0.113.10 for outbound traffic. This meets the requirement to translate RFC 1918 source addresses to a specific source IP when accessing an external FTP server, while preserving port multiplexing.
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.
✗
Policy-based NAT using an IP pool set to 'Fixed Port Range'
Why it's wrong here
Fixed port range is a type of IP pool for port allocation, not for specifying a single source IP.
✗
Virtual IP (VIP) mapping the internal server to 203.0.113.10
Why it's wrong here
VIP is for inbound DNAT (destination NAT), not for outbound source translation.
✗
Central SNAT with dynamic IP pool
Why it's wrong here
Central SNAT can use IP pools, but the question implies policy-based; both could work, but policy-based is more direct for specific destination.
✓
Policy-based NAT using an IP pool with type 'Overload' and the IP address 203.0.113.10
Why this is correct
An IP pool with type 'Overload' (PAT) using a single IP will translate all matching sessions to that IP address. This meets the requirement.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is confusing VIP (inbound destination NAT) with source NAT (SNAT), leading candidates to select Option B, even though the requirement is for outbound traffic from internal clients to use a specific source IP.
Detailed technical explanation
How to think about this question
Policy-based NAT with an 'Overload' IP pool performs Port Address Translation (PAT), mapping many internal private IPs to a single public IP by using unique source ports. This is essential for conserving public IPv4 addresses and is commonly used in enterprise edge firewalls. In real-world scenarios, FTP may require ALG inspection to handle dynamic data ports, but the NAT method itself remains PAT.
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.
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: Policy-based NAT using an IP pool with type 'Overload' and the IP address 203.0.113.10 — Option D is correct because policy-based NAT with an IP pool type 'Overload' (PAT) allows multiple internal hosts to share the single public IP 203.0.113.10 for outbound traffic. This meets the requirement to translate RFC 1918 source addresses to a specific source IP when accessing an external FTP server, while preserving port multiplexing.
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.
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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