The correct approach is to create a new central SNAT policy with the specific subnet as the source and place it above the existing policy. This works because central SNAT policies are evaluated sequentially from top to bottom, and the first matching policy is applied; by inserting a more specific policy higher in the order, you override the general rule for that subnet while leaving other traffic unaffected. On the Fortinet NSE 4 exam, this scenario tests your understanding of policy-based NAT precedence and the fact that dynamic IP pools are not supported in central SNAT—a common trap is assuming you can modify the existing policy’s source or use a dynamic pool instead. Remember the key principle: in central SNAT, order matters, and specificity wins. A simple memory tip is “specific above general, static over dynamic.”
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 has a central SNAT policy that translates internal users to a single IP pool address. The admin wants specific traffic (e.g., from a particular subnet) to use a different IP pool. What is the correct approach?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a new central SNAT policy with the specific subnet as source and place it above the existing policy
Central SNAT policies are evaluated in order; they can include source and destination criteria. To override the general policy, a more specific policy must be placed above it. Dynamic IP pools cannot be used in policy-based NAT for central SNAT.
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.
✓
Create a new central SNAT policy with the specific subnet as source and place it above the existing policy
Why this is correct
Central SNAT policies are evaluated sequentially. A more specific source policy above will match first.
Modify the existing central SNAT policy to use a dynamic IP pool
Why it's wrong here
A dynamic IP pool is the same concept; it doesn't allow selective subnet translation.
Common exam traps
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: Create a new central SNAT policy with the specific subnet as source and place it above the existing policy — Central SNAT policies are evaluated in order; they can include source and destination criteria. To override the general policy, a more specific policy must be placed above it. Dynamic IP pools cannot be used in policy-based NAT for central SNAT.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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