The answer is to use a One-to-One IP Pool or a Fixed Port Range IP Pool. A One-to-One IP Pool maps each internal IP from the 10.0.0.0/8 network to a unique public IP in the 203.0.113.1-203.0.113.10 range, which inherently preserves the original source port because no port translation is needed—each internal host gets its own dedicated external address. A Fixed Port Range IP Pool also preserves the source port by allocating a static range of ports per public IP, ensuring that the original port number is not altered for specific applications. On the Fortinet NSE 4 exam, this question tests your understanding of how different IP pool types handle source NAT, with the common trap being that Overload (PAT) pools change the source port, which would break the requirement. Remember the memory tip: “One-to-One means no port is undone; Fixed Range keeps the port unchanged.”
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 admin needs to configure source NAT for traffic from the internal network (10.0.0.0/8) to the internet. The requirement is to translate all internal IPs to a range of public IPs (203.0.113.1-203.0.113.10) while preserving the source port for specific applications. Which TWO configurations can achieve this? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a One-to-One IP Pool
Option A is correct because a One-to-One IP Pool maps each internal IP to a unique public IP from the range 203.0.113.1-203.0.113.10, preserving the original source port for each session. This meets the requirement to translate all internal IPs while keeping the source port unchanged for specific applications. The pool size (10 IPs) must be sufficient for the number of concurrent internal hosts.
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.
✓
Use a One-to-One IP Pool
Why this is correct
One-to-one maps each internal IP to a unique public IP, preserving ports.
Related concept
Read the scenario before looking for a memorised answer.
Without an IP pool, the interface IP is used with PAT, changing ports.
✗
Configure Central SNAT with Overload
Why it's wrong here
Central SNAT with overload also changes source ports.
✓
Use a Fixed Port Range IP Pool
Why this is correct
Fixed port range preserves source ports by assigning a range of ports per user.
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 that candidates often confuse 'preserving the source port' with PAT (overload) behavior, assuming any dynamic pool will work, but only One-to-One and Fixed Port Range pools avoid port translation and keep the original port intact.
Detailed technical explanation
How to think about this question
One-to-One NAT (also called static NAT) creates a fixed 1:1 mapping between private and public IPs, ensuring that the source port remains unchanged because no port translation occurs. This is defined in RFC 2663 and is ideal for applications that require port preservation, such as SIP or FTP in active mode. The Fixed Port Range IP Pool (Option E) is a variant of dynamic NAT that maps a range of private IPs to a range of public IPs without port translation, preserving the original source port as long as the number of concurrent sessions does not exceed the pool size.
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: Use a One-to-One IP Pool — Option A is correct because a One-to-One IP Pool maps each internal IP to a unique public IP from the range 203.0.113.1-203.0.113.10, preserving the original source port for each session. This meets the requirement to translate all internal IPs while keeping the source port unchanged for specific applications. The pool size (10 IPs) must be sufficient for the number of concurrent internal hosts.
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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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.
Question Discussion
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