The answer is that the firewall policy’s source or destination addresses are not correctly set to the local and remote subnets. In a policy-based IPsec VPN, the firewall policy must explicitly match the traffic flows using the correct source (192.168.1.0/24) and destination (10.0.2.0/24) addresses, even when the tunnel interface and action are set to IPSEC. Without these specific address objects, the FortiGate cannot associate the traffic with the VPN policy, causing the tunnel to remain up but traffic to fail. On the Fortinet NSE 4 exam, this scenario tests your understanding that policy-based VPNs rely on firewall policies to trigger encryption, unlike route-based VPNs. A common trap is assuming a working tunnel and correct interface selection guarantee traffic flow, but the missing address match is the culprit. Memory tip: “Policy-based VPNs need a policy that sees both ends—source and destination must match the subnets you intend to protect.”
NSE4 Authentication and VPN Practice Question
This NSE4 practice question tests your understanding of authentication and vpn. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 is configured with IPsec VPN using IKEv2 and a policy-based tunnel. The remote subnet is 10.0.2.0/24, and the local subnet is 192.168.1.0/24. The tunnel is up, but traffic from 192.168.1.0/24 to 10.0.2.0/24 fails. The administrator checks the firewall policy and sees a policy allowing traffic from the local interface (port1) to the remote interface (virtual ipsec interface) with the action set to IPSEC. What is the most likely missing configuration?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Why wrong: IKEv2 supports both route-based and policy-based VPNs.
B
The tunnel interface is not assigned to the correct VDOM
Why wrong: VDOM assignment would affect multiple features, but the tunnel is up, so VDOM is likely correct.
C
The Phase 2 proposal does not match the remote subnet
Why wrong: Phase 2 proposals define encryption/authentication, not subnets. Subnet mismatch would be in Phase 2 selectors.
D
The firewall policy's source or destination addresses are not correctly set to the local and remote subnets
Policy-based VPNs require the policy to explicitly specify the local and remote subnets in source/destination. If set to 'all', it may work, but the failure suggests mismatch.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The firewall policy's source or destination addresses are not correctly set to the local and remote subnets
Policy-based VPNs require a firewall policy that matches the traffic and has the action set to IPSEC. However, the 'incoming interface' must be the internal interface (port1) and 'outgoing interface' the tunnel interface. The description suggests both are correct, but the missing piece is that the 'source' and 'destination' addresses in the policy must match the local and remote subnets. The policy likely uses all addresses or incorrect subnets.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
IKEv2 does not support policy-based VPNs
Why it's wrong here
IKEv2 supports both route-based and policy-based VPNs.
✗
The tunnel interface is not assigned to the correct VDOM
Why it's wrong here
VDOM assignment would affect multiple features, but the tunnel is up, so VDOM is likely correct.
✗
The Phase 2 proposal does not match the remote subnet
Why it's wrong here
Phase 2 proposals define encryption/authentication, not subnets. Subnet mismatch would be in Phase 2 selectors.
✓
The firewall policy's source or destination addresses are not correctly set to the local and remote subnets
Why this is correct
Policy-based VPNs require the policy to explicitly specify the local and remote subnets in source/destination. If set to 'all', it may work, but the failure suggests mismatch.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
Authentication and VPN — This question tests Authentication and VPN — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The firewall policy's source or destination addresses are not correctly set to the local and remote subnets — Policy-based VPNs require a firewall policy that matches the traffic and has the action set to IPSEC. However, the 'incoming interface' must be the internal interface (port1) and 'outgoing interface' the tunnel interface. The description suggests both are correct, but the missing piece is that the 'source' and 'destination' addresses in the policy must match the local and remote subnets. The policy likely uses all addresses or incorrect subnets.
What should I do if I get this NSE4 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.