- A
The FortiGate uses the pool IPs in round-robin and assigns the same port number as the original source port
Why wrong: Fixed port range modifies the port to be within the configured range.
- 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
Fixed Port Range maps the internal IP/port to a consistent public IP and port range, ensuring that the same internal host uses the same public IP (if possible) and port range.
- C
The FortiGate randomly selects an IP from the pool and a random port from 10000-20000 for each session
Why wrong: Fixed Port Range does not use random ports; it uses a hash or sequential assignment.
- D
The FortiGate uses the first available IP in the pool and assigns a port sequentially from 10000 upward
Why wrong: This describes overload behavior, not fixed port range.
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 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?
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 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
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.
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 FortiGate uses the pool IPs in round-robin and assigns the same port number as the original source port
Why it's wrong here
Fixed port range modifies the port to be within the configured range.
- ✓
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
Why this is correct
Fixed Port Range maps the internal IP/port to a consistent public IP and port range, ensuring that the same internal host uses the same public IP (if possible) and port range.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The FortiGate randomly selects an IP from the pool and a random port from 10000-20000 for each session
Why it's wrong here
Fixed Port Range does not use random ports; it uses a hash or sequential assignment.
- ✗
The FortiGate uses the first available IP in the pool and assigns a port sequentially from 10000 upward
Why it's wrong here
This describes overload behavior, not fixed port range.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'Fixed Port Range' with 'Port Block Allocation' or assume it behaves like standard dynamic PAT (Port Address Translation), where each session gets a random or sequential port, but Fixed Port Range is specifically designed for deterministic mapping per internal host.
Detailed technical explanation
How to think about this question
Under the hood, Fixed Port Range NAT uses a hash of the internal source IP (and optionally the source port) to select a public IP from the pool and then maps that internal host to a contiguous block of ports within the configured range (e.g., 10000-20000). This is particularly important for applications like SIP or ALGs that embed IP addresses and ports in the payload, as the external server expects the same source IP and port range for all sessions from the same internal host. The FortiGate also supports 'Fixed Port Range' with 'Permit Any' to allow multiple internal hosts to share the same public IP but with different port ranges.
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.
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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 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 — 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.
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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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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