This NSE4 practice question tests your understanding of firewall policies and nat. 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.
An admin needs to allow inbound SMTP traffic from the internet to a mail server in the DMZ. The public IP is 203.0.113.10, and the mail server's private IP is 10.0.0.5. Which VIP configuration is correct?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VIP: external IP 203.0.113.10 port 25 -> internal IP 10.0.0.5 port 25
Option A is correct because it configures a Virtual IP (VIP) that maps the public IP 203.0.113.10 on TCP port 25 (SMTP) to the internal mail server IP 10.0.0.5 on port 25. This allows inbound SMTP traffic from the internet to reach the mail server in the DMZ, performing both destination NAT (DNAT) and port forwarding for the specific SMTP service.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
VIP: external IP 203.0.113.10 port 25 -> internal IP 10.0.0.5 port 25
Why this is correct
This correctly maps SMTP traffic to the mail server.
Related concept
Read the scenario before looking for a memorised answer.
✗
VIP: external IP 203.0.113.10 port 25 -> internal IP 10.0.0.5 port 80
Why it's wrong here
This maps SMTP to HTTP port on the server, which would not work.
✗
VIP: external IP 203.0.113.10 all ports -> internal IP 10.0.0.5 all ports
Why it's wrong here
Mapping all ports is not recommended and may expose unnecessary services.
✗
VIP: external IP 203.0.113.10 port 80 -> internal IP 10.0.0.5 port 80
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse port numbers or assume that any port mapping will work, but the NSE4 exam specifically tests that the VIP must match the service port (SMTP = 25) and that only the correct port mapping enables the intended application traffic.
Detailed technical explanation
How to think about this question
In FortiGate, a Virtual IP (VIP) object performs Destination NAT (DNAT) by translating the destination IP and optionally the port of incoming packets. For SMTP, TCP port 25 is the standard port defined in RFC 5321. The VIP configuration must match the service port exactly; using a different port or a wildcard mapping would either misdirect traffic or create a security risk. In real-world deployments, you would also need a firewall policy allowing traffic from the WAN to the VIP on port 25 to complete the configuration.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: VIP: external IP 203.0.113.10 port 25 -> internal IP 10.0.0.5 port 25 — Option A is correct because it configures a Virtual IP (VIP) that maps the public IP 203.0.113.10 on TCP port 25 (SMTP) to the internal mail server IP 10.0.0.5 on port 25. This allows inbound SMTP traffic from the internet to reach the mail server in the DMZ, performing both destination NAT (DNAT) and port forwarding for the specific SMTP service.
What should I do if I get this NSE4 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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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.
Question Discussion
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