The correct answer is that the IP pool's port range is exhausted because the number of internal hosts exceeds the number of available port ranges. A Fixed Port Range IP pool assigns a unique, dedicated port range to each internal source IP, meaning that with a /28 subnet (16 IPs) and a single port range (10000-20000), only 16 internal hosts can be supported at once. When the internal network has more than 16 active hosts, the pool runs out of ranges, causing connection failures for any additional hosts. On the Fortinet NSE 4 exam, this scenario tests your understanding of how Fixed Port Range differs from Overload or Port Block Allocation—a common trap is assuming all IP pool types share ports dynamically. Remember the key distinction: Fixed Port Range is a one-to-one mapping of internal IP to a port range, so exhaustion occurs when the host count exceeds the pool size. Memory tip: "Fixed means fixed per host—count your IPs, not your ports."
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 admin configures an IP pool with type 'Fixed Port Range' to translate source IPs from 192.168.1.0/24 to 203.0.113.0/28 using port range 10000-20000. After applying the IP pool to a policy, some users cannot establish connections while others work. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The IP pool's port range is exhausted because the number of internal hosts exceeds the number of available port ranges
Fixed port range assigns a unique port range per internal IP. If the number of internal hosts exceeds the number of available port ranges, no range will be available for new hosts, causing failures.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 internal subnet is using RFC 1918 addresses that cannot be NATed
Why it's wrong here
RFC 1918 addresses can be NATed.
✓
The IP pool's port range is exhausted because the number of internal hosts exceeds the number of available port ranges
Why this is correct
Fixed port range provides a dedicated port block per host; if more hosts than port blocks, some will be denied.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The IP pool's port range is exhausted because the number of internal hosts exceeds the number of available port ranges — Fixed port range assigns a unique port range per internal IP. If the number of internal hosts exceeds the number of available port ranges, no range will be available for new hosts, causing failures.
What should I do if I get this NSE4 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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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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