Quick Answer
The answer is source interface, destination interface, and service set to SMTP. This configuration is correct because a firewall policy must define the traffic’s origin and destination interfaces—internal and DMZ respectively—to control the flow between network zones, while the service setting restricts the policy to TCP port 25, ensuring only SMTP traffic is permitted. On the Fortinet NSE 4 exam, this tests your understanding of policy elements beyond address objects; a common trap is assuming source and destination addresses are mandatory settings when the question specifically asks for interface and service settings. The scenario also reinforces that blocking internet access to the mail server is handled by the absence of a policy from DMZ to WAN, not by settings within this internal-to-DMZ rule. Remember the mnemonic “ISD” for Interface, Service, Destination interface—the three core policy settings that define traffic flow.
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 create a firewall policy that allows SMTP traffic (TCP/25) from the internal network (10.0.0.0/24) to a mail server in the DMZ (172.16.1.10). Additionally, the admin wants to ensure that the mail server can only be accessed by the internal network, not from the internet. Which THREE settings must be configured in the firewall policy? (Choose three.)
Source interface set to 'internal'
Defines where traffic originates.
Set schedule to 'always'
Why wrong: Schedule is optional.
Destination interface set to 'dmz'
Defines where traffic goes.
Service set to 'SMTP'
Defines the allowed protocol/port.
Enable NAT to translate source IP
Why wrong: NAT is not required for DMZ access.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Source interface set to 'internal'
To define the traffic, the policy needs source interface (internal), destination interface (DMZ), source address (internal subnet), and destination address (mail server). Also, the service must be SMTP. Additionally, to block internet access, the policy from DMZ to internet should not exist, but the question asks for settings in the policy for internal traffic. The three required are source interface, destination interface, and service. Address objects are also required but the question asks for settings, not objects. However, the options include interface and service settings.
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
For each option: why learners choose it and why it is or isn't the right answer here.
Source interface set to 'internal'
Why this is correct
Defines where traffic originates.
Related concept
CIDR notation defines the prefix length.
Set schedule to 'always'
Why it's wrong here
Schedule is optional.
Destination interface set to 'dmz'
Why this is correct
Defines where traffic goes.
Related concept
CIDR notation defines the prefix length.
Service set to 'SMTP'
Why this is correct
Defines the allowed protocol/port.
Related concept
CIDR notation defines the prefix length.
Enable NAT to translate source IP
Why it's wrong here
NAT is not required for DMZ access.
Common exam traps
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
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.
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
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
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.
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FAQ
Firewall Policies and NAT — This question tests Firewall Policies and NAT — CIDR notation defines the prefix length..
The correct answer is: Source interface set to 'internal' — To define the traffic, the policy needs source interface (internal), destination interface (DMZ), source address (internal subnet), and destination address (mail server). Also, the service must be SMTP. Additionally, to block internet access, the policy from DMZ to internet should not exist, but the question asks for settings in the policy for internal traffic. The three required are source interface, destination interface, and service. Address objects are also required but the question asks for settings, not objects. However, the options include interface and service settings.
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.
CIDR notation defines the prefix length.
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Last reviewed: Jun 21, 2026
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