The correct answer is to use address objects that contain multiple subnets in the Phase2 definition, or to configure multiple Phase2 selectors. This works because IPsec Phase2 defines the traffic selectors—the specific local and remote subnets that will be encrypted through the tunnel. When you need to support multiple subnets behind each FortiGate, you must either create a separate Phase2 entry for each subnet pair, or leverage modern FortiOS capabilities by using a single Phase2 that references address objects containing multiple subnet entries. On the Fortinet NSE 4 Network Security Professional exam, this question tests your understanding of IKEv1 main mode and the flexibility of Phase2 selectors; a common trap is assuming you must always use one Phase2 per subnet, but FortiOS now allows consolidated configurations. Remember the memory tip: “One Phase2 per pair, or group them with an address object for care.”
NSE4 Authentication and VPN Practice Question
This NSE4 practice question tests your understanding of authentication and vpn. 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 administrator is configuring an IPsec VPN between two FortiGates using IKEv1. The tunnel must use main mode and support multiple subnets behind each gate. Which Phase2 settings are required to allow multiple subnets? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create multiple Phase2 selectors, each with different local and remote subnets
To support multiple subnets, you can either configure multiple Phase2 selectors (one per subnet pair) or define the local/remote subnets in the Phase2 configuration. Modern FortiOS allows multiple subnets in a single Phase2. Option B is correct (multiple Phase2 entries), and Option D (using address objects with multiple addresses) is also correct.
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.
✗
Set the Phase2 keylife to a higher value
Why it's wrong here
Keylife does not affect subnet support.
✗
Set the Phase2 proposal to include multiple encryption algorithms
Why it's wrong here
Multiple encryption algorithms are for proposal matching, not multiple subnets.
✓
Create multiple Phase2 selectors, each with different local and remote subnets
Why this is correct
Each Phase2 selector defines a single traffic pair; multiple selectors cover multiple subnets.
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: Create multiple Phase2 selectors, each with different local and remote subnets — To support multiple subnets, you can either configure multiple Phase2 selectors (one per subnet pair) or define the local/remote subnets in the Phase2 configuration. Modern FortiOS allows multiple subnets in a single Phase2. Option B is correct (multiple Phase2 entries), and Option D (using address objects with multiple addresses) is also correct.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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