Question 1mediummultiple choice
Read the full NAT/PAT explanation →Question 225 of 1,000
Quick Answer
The answer is that the IP pool used for SNAT has exhausted its address range. When a FortiGate runs out of available translated IPs in the pool, new outbound sessions cannot complete the source NAT process, leaving them stuck in a pending state. The `diagnose sys session list` output reveals this through sessions showing `proto_state=01` with a short TTL, which indicates the TCP handshake never finished because no NAT address could be assigned. On the Fortinet NSE 4 exam, this scenario tests your understanding of how SNAT pool depletion manifests in session table diagnostics, often disguised as intermittent connectivity issues that users report. A common trap is to misdiagnose this as a routing or firewall policy problem, but the short TTL and proto_state=01 are the key clues pointing to pool exhaustion. Memory tip: think of the IP pool as a parking lot—when all spaces are taken, new cars (sessions) have to wait or leave, and the session table shows them as incomplete.
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.
An administrator configures a firewall policy allowing traffic from the internal network to the internet with NAT enabled. Users report that some outbound connections fail intermittently. The administrator runs 'diagnose sys session list' and sees many sessions in 'proto_state=01' with a short TTL. 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.
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 IP pool used for SNAT has exhausted its address range
The 'diagnose sys session list' output showing many sessions in 'proto_state=01' with a short TTL indicates that sessions are failing to establish properly. When the IP pool used for Source NAT (SNAT) exhausts its address range, new outbound connections cannot obtain a translated source IP, causing them to fail intermittently. This matches the symptom of intermittent failures as the pool becomes temporarily depleted.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may misinterpret 'proto_state=01' as a protocol or state machine error, rather than recognizing it as a symptom of NAT resource exhaustion, leading them to incorrectly select options related to policy misconfiguration or security profiles.
Detailed technical explanation
How to think about this question
In FortiOS, 'proto_state=01' often corresponds to a session in the initial SYN state (TCP state 0x01 for SYN_SENT). When the IP pool is exhausted, the FortiGate cannot allocate a translated IP for new sessions, causing them to be dropped or fail to progress beyond the initial handshake, resulting in a short TTL. The 'diagnose sys session list' command is a key troubleshooting tool for identifying NAT pool exhaustion by observing many sessions stuck in early states with low TTL values.
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 IP pool used for SNAT has exhausted its address range — The 'diagnose sys session list' output showing many sessions in 'proto_state=01' with a short TTL indicates that sessions are failing to establish properly. When the IP pool used for Source NAT (SNAT) exhausts its address range, new outbound connections cannot obtain a translated source IP, causing them to fail intermittently. This matches the symptom of intermittent failures as the pool becomes temporarily depleted.
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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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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