Quick Answer
The answer is the primary IP address of the egress interface, port2 (203.0.113.1). When source NAT is configured with the egress interface IP, the FortiGate performs dynamic PAT, replacing the internal source IP (10.0.1.10) with the interface’s primary address before forwarding the packet out port2. This is the default behavior when the firewall policy’s source NAT is set to “Use Outgoing Interface Address,” ensuring the web server at 198.51.100.1 sees all traffic as originating from the FortiGate’s public IP. On the Fortinet NSE 7 Advanced Security NSE7 exam, this concept tests your understanding of NAT modes and interface-based translation—a common trap is assuming the source IP becomes the destination server’s IP or a pool address. Remember: source NAT with egress interface IP always uses the interface’s primary IP, not a secondary or virtual IP. Memory tip: “Egress equals address—the packet leaves wearing the interface’s coat.”
This NSE7 practice question tests your understanding of enterprise firewall and vdoms. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 in NAT mode has a VDOM with interface port1 (10.0.1.0/24) and port2 (203.0.113.0/24). A policy allows traffic from port1 to port2 with source NAT using the IP of port2. A user at 10.0.1.10 initiates a connection to a web server at 198.51.100.1. What will be the source IP after NAT?
A random IP from the port2 subnet
Why wrong: Source NAT uses the interface IP unless a different IP is specified.
10.0.1.10
Why wrong: That's the original source before NAT.
198.51.100.1
Why wrong: That's the destination.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The IP address of port2 (e.g., 203.0.113.1)
When source NAT is configured to use the IP address of the egress interface (port2), the FortiGate performs dynamic PAT (Port Address Translation) and translates the source IP of the packet to the primary IP address of port2 (203.0.113.1). This is the default behavior when 'set srcaddr' is set to the interface IP in the firewall policy. The user at 10.0.1.10 will therefore appear to the web server at 198.51.100.1 as coming from 203.0.113.1.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Common exam traps
The trap here is that candidates often assume source NAT uses a random IP from the subnet (Option B) or forget that the source IP must be the egress interface IP, leading them to select the original private IP (Option C) or the destination IP (Option D).
Detailed technical explanation
Under the hood, FortiGate uses a session-based NAT table where the source IP and port are replaced with the egress interface IP and a dynamically allocated high port (e.g., 203.0.113.1:54321). This PAT behavior is defined in RFC 2663 and is essential for conserving public IPv4 addresses. In a VDOM environment, the NAT is performed within the VDOM's routing context, and the translated packet exits via port2 with the source IP set to the interface's primary address, not a secondary or virtual IP.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Enterprise Firewall and VDOMs — This question tests Enterprise Firewall and VDOMs — Read the scenario before looking for a memorised answer..
The correct answer is: The IP address of port2 (e.g., 203.0.113.1) — When source NAT is configured to use the IP address of the egress interface (port2), the FortiGate performs dynamic PAT (Port Address Translation) and translates the source IP of the packet to the primary IP address of port2 (203.0.113.1). This is the default behavior when 'set srcaddr' is set to the interface IP in the firewall policy. The user at 10.0.1.10 will therefore appear to the web server at 198.51.100.1 as coming from 203.0.113.1.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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