NSE4 Firewall Policies and NAT Practice Question
An administrator configures a Central SNAT policy to translate traffic from the internal network (10.0.0.0/8) to the internet using the IP pool 'pool1'. The administrator also has a firewall policy that uses policy-based NAT with an IP pool 'pool2'. Both policies match the same traffic. Which NAT will be applied?
⚠ Common exam trap
It's easy for candidates to assume policy-based NAT within a firewall policy takes precedence because it is more specific, but FortiOS gives Central SNAT higher priority regardless of specificity.
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
✓
Central SNAT using pool1
Central SNAT policies have higher priority than policy-based NAT when both match the same traffic. In FortiOS, Central SNAT is evaluated before firewall policies, and if a match is found, the policy-based NAT within the firewall policy is ignored. Therefore, pool1 is applied.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Central SNAT using pool1
Why this is correct
Central SNAT rules are evaluated before any policy-based NAT. In FortiGate's NAT execution order, central source NAT takes precedence over the NAT settings configured inside a firewall policy, so a matching central SNAT rule using pool1 is selected. The session's source IP is translated to an address from pool1, and the policy's NAT configuration is completely bypassed.
- ✗
Both NAT rules are applied in sequence
Why it's wrong here
FortiGate never applies central SNAT and policy-based NAT in sequence for the same session; only one source NAT action is executed. The NAT lookup process stops at the first matching rule that has highest precedence, so if the central SNAT rule matches, it is the sole translation. Applying both would result in overlapping or cascading source IP changes, which is not how FortiGate's NAT logic works. Thus, pool2 is never used as a second hop.
- ✗
The traffic is dropped due to conflicting NAT configurations
Why it's wrong here
A conflict between central SNAT and policy-based NAT does not cause FortiGate to drop traffic, because precedence rules resolve the selection deterministically. FortiGate does not treat overlapping NAT configurations as an error; it simply continues evaluating and picks the highest-priority match. Since the central SNAT rule wins, the session is forwarded normally with pool1 as the source IP, and no drop occurs due to any perceived mismatch.
- ✗
Policy-based NAT using pool2
Why it's wrong here
Policy-based NAT using pool2 would only apply if no central NAT rule matched the session, but here the central SNAT rule matches the traffic. FortiGate processes central NAT before policy-level NAT, so the central SNAT rule is selected and the policy's NAT setting is ignored. Therefore, even if the outgoing interface policy has 'Use Outgoing Interface Address' or a dynamic pool like pool2 configured, it is overridden by the central rule.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.