Using IP Pools to Assign a Specific Source NAT IP
An administrator has configured a firewall policy that allows outbound traffic from a subnet to the internet, with NAT enabled. The external IP is 203.0.113.1. However, the administrator wants all traffic from a specific internal server (10.0.0.10) to appear with source IP 203.0.113.2. What should the administrator do?
Quick Answer
The correct answer is to create an IP Pool with 203.0.113.2 and reference it in the policy. This works because a standard firewall policy with NAT enabled uses the egress interface IP for source translation, but when you need a specific source NAT IP for traffic from a particular host, you must define an IP Pool object containing that exact address and then apply it within the policy’s NAT settings. On the Fortinet NSE 4 exam, this scenario tests your understanding of Dynamic IP Pool versus Fixed Port Address Translation, and the common trap is confusing the pool with the interface IP or assuming a secondary IP on the interface will suffice. Remember that IP Pools override the default interface IP for NAT, and the pool must be explicitly referenced in the policy—simply adding the IP to the interface does not change the translated source. A useful memory tip: “Pool for a specific rule, not for the whole interface.”
⚠ Common exam trap
Many candidates confuse Virtual IP (VIP) with IP Pool: VIP is for destination NAT (inbound), while IP Pool is for source NAT (outbound), and many candidates mistakenly apply a VIP to change the source IP of outbound traffic.
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
✓
Create an IP Pool with 203.0.113.2 and reference it in the policy
An IP Pool in FortiGate allows you to override the source IP address for specific traffic, even when NAT is enabled on the policy. By creating an IP Pool with the single address 203.0.113.2 and referencing it in the firewall policy, traffic from 10.0.0.10 will be NATed to that specific IP instead of the outgoing interface address (203.0.113.1). This is the standard method for fixed source NAT (also called 'static NAT' or 'one-to-one NAT') for a specific host.
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 VIP with the external IP and apply it to the policy
Why it's wrong here
VIP is for destination NAT.
- ✓
Create an IP Pool with 203.0.113.2 and reference it in the policy
Why this is correct
An IP Pool allows selecting a different source NAT IP.
- ✗
Configure route-based NAT
Why it's wrong here
Route-based NAT is not a standard term; policy-based NAT is used.
- ✗
Set the NAT to 'Use Outgoing Interface Address'
Why it's wrong here
That uses the egress interface IP (203.0.113.1), not 203.0.113.2.
Visual reference
Go deeper
Related to this question
About these practice questions
Courseiva writes every NSE4 question from scratch — 282 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more way this is tested on NSE4
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A FortiGate administrator needs to create a firewall policy that allows traffic from the internal network (10.0.0.0/8) to a public web server (203.0.113.10) on port 443. The policy must also perform source NAT using the FortiGate's external IP (198.51.100.1). Which NAT configuration should be applied?
medium- A.Create an IP pool with the external IP and reference it in the firewall policy
- ✓ B.Enable NAT on the firewall policy without specifying an IP pool
- C.Create a VIP for the web server and reference it in the firewall policy
- D.Configure Central SNAT and a matching rule
Why B: When a firewall policy uses source NAT (SNAT) to translate internal source IPs to the FortiGate's own egress interface IP, simply enabling NAT on the policy without specifying an IP pool is sufficient. This is the default behavior: the FortiGate automatically performs source NAT using the IP address of the outgoing interface (in this case, 198.51.100.1). No additional IP pool or central NAT rule is required for this standard outbound NAT scenario.
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.