How to Configure Source NAT to Use Interface IP on FortiGate
A FortiGate in NAT/Route mode has a policy with NAT enabled. The admin needs the source IP of traffic from internal users (192.168.1.0/24) to be translated to the interface IP of port1 (203.0.113.1) when accessing the internet. Which configuration is necessary?
Quick Answer
The answer is to configure a firewall policy with NAT enabled and the outbound interface set to port1. This works because in NAT/Route mode, enabling NAT on a policy automatically performs source NAT (SNAT) using the IP address of the specified outbound interface—in this case, 203.0.113.1—without needing an IP pool or static route for the translation itself. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of policy-based NAT behavior, where the default action is to translate all internal source addresses (like 192.168.1.0/24) to the interface IP when NAT is checked. A common trap is assuming you must create a separate IP pool or configure Central NAT, but the exam emphasizes that the interface IP is the default translation target for a policy with NAT enabled and an outbound interface set. Remember the memory tip: “NAT on the policy, interface IP is the trick”—if the outbound interface is defined, the FortiGate uses its IP automatically, no pool required.
⚠ Common exam trap
Watch out — candidates often confuse the need for an IP pool or static route with the simple policy-based NAT, assuming that translating to the interface IP requires additional configuration beyond enabling NAT on the policy.
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
✓
Configure a firewall policy with NAT enabled and the outbound interface set to port1
In NAT/Route mode, enabling NAT on a firewall policy with the outbound interface set to port1 causes the FortiGate to translate the source IP of traffic from the internal network (192.168.1.0/24) to the IP address of that interface (203.0.113.1) by default. This is the standard method for source NAT (SNAT) in a policy-based configuration, requiring no additional IP pool or static route for the translation itself.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Add a static route for 192.168.1.0/24 with next-hop 203.0.113.1
Why it's wrong here
Static routes do not affect NAT translation.
- ✗
Set the administrative access to HTTPS on port1
Why it's wrong here
Administrative access does not affect NAT.
- ✗
Create a central NAT rule with source 192.168.1.0/24 and IP pool 203.0.113.2-203.0.113.10
Why it's wrong here
This uses a pool, not the interface IP.
- ✓
Configure a firewall policy with NAT enabled and the outbound interface set to port1
Why this is correct
By default, NAT on a policy uses the egress interface IP as the translated source.
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1 more way this is tested on NSE4
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Variation 1. A FortiGate is set to NAT/Route mode. The admin wants traffic from internal users to the internet to use an IP address on the WAN interface for source NAT. Which configuration is required?
medium- A.Set the FortiGate to transparent mode
- B.Configure a policy route to force traffic through a specific interface
- C.Configure a virtual IP mapping internal IPs to the WAN IP
- ✓ D.Enable NAT on the policy from internal to WAN and set the outgoing interface to the WAN interface
Why D: In NAT/Route mode, source NAT (SNAT) is configured by enabling NAT on the firewall policy that governs traffic from the internal network to the WAN interface. When NAT is enabled on the policy and the outgoing interface is set to the WAN interface, FortiGate automatically translates the source IP of internal users to the primary IP address of that WAN interface (or a configured IP pool). This is the standard method for allowing internal users to access the internet with a public IP address.
JA
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.