The answer is a fixed port range that is too small, causing port exhaustion. When a FortiGate uses a Fixed Port Range IP pool for policy-based NAT, each IP address in the pool is assigned a specific, limited set of source ports for translation. Once all ports within that range are consumed—indicated by the diagnostic output showing `used_port=65535`—no new outbound sessions can be initiated to the target server, as no port is available for the NAT mapping. On the Fortinet NSE 4 exam, this scenario tests your understanding of how IP pool types behave under load; a common trap is confusing Fixed Port Range with Overload (PAT) pools, which reuse ports dynamically. Remember the memory tip: “Fixed means finite—when the port well runs dry, new connections die.”
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. 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 administrator configures a policy-based NAT using an IP pool with type 'Fixed Port Range' for internal users accessing a specific external server. Users report that after some time, they cannot establish new connections to the server. 'diagnose ip pool list' shows many entries with 'used_port=65535'. 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.
Why wrong: Server-side limitations would not cause local port allocation to show 'used_port=65535' in the IP pool.
B
The firewall policy has a timeout setting that is too low
Why wrong: Timeout settings affect session persistence, not port allocation exhaustion.
C
The IP pool has run out of IP addresses
Why wrong: The output shows port exhaustion, not IP exhaustion. Fixed port range uses per-IP port ranges.
D
The fixed port range is too small, causing port exhaustion
Fixed port range NAT assigns a block of ports to each source IP. If the range is small (e.g., 1 port), it fills quickly and blocks new sessions. The 'used_port=65535' indicates the last port in a range is in use.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The fixed port range is too small, causing port exhaustion
The 'Fixed Port Range' IP pool type allocates a specific range of ports per IP address for NAT translations. When all ports in the range are exhausted (indicated by 'used_port=65535'), no new connections can be established, causing the reported issue. This is classic port exhaustion, making D correct.
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 external server is rate-limiting connections
Why it's wrong here
Server-side limitations would not cause local port allocation to show 'used_port=65535' in the IP pool.
✗
The firewall policy has a timeout setting that is too low
Why it's wrong here
Timeout settings affect session persistence, not port allocation exhaustion.
✗
The IP pool has run out of IP addresses
Why it's wrong here
The output shows port exhaustion, not IP exhaustion. Fixed port range uses per-IP port ranges.
✓
The fixed port range is too small, causing port exhaustion
Why this is correct
Fixed port range NAT assigns a block of ports to each source IP. If the range is small (e.g., 1 port), it fills quickly and blocks new sessions. The 'used_port=65535' indicates the last port in a range is in use.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse IP address exhaustion (Option C) with port exhaustion, but the diagnostic output clearly shows IP addresses are still available while ports are maxed out, pointing directly to the fixed port range being too small.
Trap categories for this question
Command / output trap
Server-side limitations would not cause local port allocation to show 'used_port=65535' in the IP pool.
Detailed technical explanation
How to think about this question
In policy-based NAT with 'Fixed Port Range', each internal IP is mapped to a single external IP with a limited set of source ports (e.g., 1024-65535). When all ports are consumed, the NAT device cannot create new sessions until existing ones time out or are released. The 'used_port=65535' value in the diagnostic output indicates that the last available port in the range has been allocated, confirming exhaustion. This is a common issue in environments with many concurrent connections, such as web scraping or high-traffic applications.
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: The fixed port range is too small, causing port exhaustion — The 'Fixed Port Range' IP pool type allocates a specific range of ports per IP address for NAT translations. When all ports in the range are exhausted (indicated by 'used_port=65535'), no new connections can be established, causing the reported issue. This is classic port exhaustion, making D correct.
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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