NSE4 Firewall Policies and NAT Practice Question
A FortiGate admin wants to create a firewall policy that allows traffic from the internal network to the internet. The source is a subnet 192.168.1.0/24, and the destination is 'all'. The admin wants to apply NAT to hide internal IPs. Which NAT configuration is BEST suited for this scenario?
⚠ Common exam trap
Watch out — candidates often confuse VIPs (destination NAT) with source NAT, or assume that a one-to-one IP pool is required for hiding internal IPs, when in fact interface NAT with PAT is the default and best practice for internet-bound traffic in FortiGate.
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
✓
Enable 'NAT' on the firewall policy and use the outgoing interface address
Enabling NAT on the firewall policy with the outgoing interface address is the standard method for source NAT (masquerading) in FortiGate. This configuration translates all internal source IPs (192.168.1.0/24) to the single IP address of the egress interface, hiding the internal subnet from the internet. It is the simplest and most efficient approach for typical internet-bound traffic, requiring no additional objects like IP pools or VIPs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure a VIP for source NAT
Why it's wrong here
A Virtual IP (VIP) is designed for destination NAT: it translates an incoming packet's destination address from a public IP to a private server address. It does not alter the source address of outbound traffic, so configuring a VIP for source NAT would have no effect on hiding internal IPs. Since the goal is to hide internal sources on egress, a VIP is the wrong tool.
- ✗
Use policy-based routing to send traffic through a NAT device
Why it's wrong here
Policy-based routing (PBR) selects a next-hop or outgoing interface based on packet attributes, but it performs no address translation whatsoever. Directing traffic to an external NAT device would require that device to actually perform the translation, and the FortiGate itself would still need a separate NAT configuration. This approach adds network complexity and does not solve the requirement of hiding internal IPs on the FortiGate, so it is incorrect.
- ✓
Enable 'NAT' on the firewall policy and use the outgoing interface address
Why this is correct
Enabling the NAT option on a firewall policy is the standard policy-based source NAT method. When NAT is enabled and no IP pool is selected, the FortiGate automatically uses the IP address of the outgoing interface as the translated source address for all matching sessions. This many-to-one hiding mechanism is the simplest and most common way to conceal all internal private IPs behind a single public address.
- ✗
Create a one-to-one IP pool and apply it to the policy
Why it's wrong here
A one-to-one IP pool reserves a unique public IP address for each internal source IP, meaning every internal host is mapped to a distinct external address. This approach does hide the original private IP, but it consumes a public IP per concurrent host and does not provide the 'many-to-one' address hiding that is typically intended. For simply hiding all internal IPs behind a single egress address, a one-to-one pool is wasteful and unnecessarily restrictive.
Visual reference
Go deeper
Related to this question
About these practice questions
One of 773 original NSE4 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
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.