The answer is that the IP pool is configured with type fixed port range, which restricts translation to a single IP address regardless of the pool size. In fixed port range mode, FortiGate assigns one IP per session and exhausts its port range before moving to the next IP, so if only one session is active or the pool is misapplied, all traffic appears from the first address. This concept tests your understanding of FortiGate IP pool types and behavior on the NSE4 exam, where overload mode distributes traffic across multiple IPs using port address translation, while fixed port range and one-to-one behave differently. A common trap is assuming a pool of multiple IPs automatically load-balances—it does not unless overload is selected. Memory tip: “Fixed locks one, Overload spreads the load, One-to-One maps a single IP.”
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 has a policy that matches traffic from LAN to WAN with NAT enabled and an IP pool. The pool contains IPs 203.0.113.1 to 203.0.113.5. The administrator notices that all traffic appears to come from 203.0.113.1. Which THREE reasons could explain this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Only one source IP is generating traffic
If all traffic uses the first IP in the pool, possible reasons: the pool is configured for fixed port range which might allocate only one IP, or the NAT mode is not overload (which uses multiple IPs), or a single session is using all ports.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Only one source IP is generating traffic
Why this is correct
If only one client, all traffic will use that client's translation.
Related concept
Static NAT maps one inside address to one outside address.
✓
The IP pool is configured with 'type one-to-one'
Why this is correct
One-to-one uses a single mapping per source IP.
Related concept
Static NAT maps one inside address to one outside address.
✗
The IP pool is configured with 'type overload' and all source ports are being used
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Only one source IP is generating traffic — If all traffic uses the first IP in the pool, possible reasons: the pool is configured for fixed port range which might allocate only one IP, or the NAT mode is not overload (which uses multiple IPs), or a single session is using all ports.
What should I do if I get this NSE4 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A FortiGate has a firewall policy with NAT enabled using an IP pool of type 'Fixed Port Range'. The pool range is 203.0.113.10-203.0.113.20 with port range 10000-20000. A user initiates a connection to an external server. Which of the following describes how the FortiGate will assign the source address and port?
hard
A.The FortiGate uses the pool IPs in round-robin and assigns the same port number as the original source port
✓ B.The FortiGate assigns a fixed IP and port mapping based on the original source IP and port, so the same internal host always gets the same public IP and port range
C.The FortiGate randomly selects an IP from the pool and a random port from 10000-20000 for each session
D.The FortiGate uses the first available IP in the pool and assigns a port sequentially from 10000 upward
Why B: Fixed Port Range NAT (also known as NAT with fixed port range) creates a deterministic mapping between an internal host's source IP and port and a specific public IP and port range from the pool. This ensures that the same internal host always receives the same public IP and a dedicated port range (10000-20000 in this case), which is essential for protocols that require consistent source addressing, such as SIP or H.323. The FortiGate does not round-robin, randomly assign, or sequentially assign ports; it uses a hash of the original source IP to select the fixed public IP and port range.
Variation 2. A FortiGate has a policy that enables NAT with an IP pool that uses overload (port address translation). The administrator notices that some applications are failing because they require a fixed source port range. What should the administrator do to resolve this?
hard
✓ A.Change the IP pool type to 'Fixed Port Range'
B.Disable NAT and use policy-based routing
C.Use Central SNAT instead of policy-based NAT
D.Enable 'Preserve Source Port' in the firewall policy
Why A: The IP pool overload mode uses dynamic port allocation, which can break applications needing a consistent port range. The 'Fixed Port Range' option in the IP pool configuration assigns a fixed port range to each session, preserving the original source port or a fixed range. Option A is correct.
Last reviewed: Jun 21, 2026
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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.
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