- A
Deploy Azure Application Gateway in front of each tier. Use WAF policies to filter traffic.
Why wrong: Application Gateway is for inbound traffic, not inter-tier.
- B
Use network security groups (NSGs) on each subnet to allow/deny traffic by IP and port. Enable NSG flow logs for auditing.
Why wrong: NSGs do not support L7 filtering and flow logs don't capture denied traffic.
- C
Deploy Azure Firewall in a hub virtual network. Use application rules to allow specific FQDNs between tiers. Enable diagnostic logs and send them to a Log Analytics workspace.
Provides L7 filtering and logging.
- D
Deploy a third-party NVA (e.g., Palo Alto) in the hub. Configure inter-tier routing through the NVA.
Why wrong: NVAs are valid but add complexity and cost; Azure Firewall is simpler.
Quick Answer
The answer is to deploy Azure Firewall in a hub virtual network with application rules and diagnostic logging. This design is correct because Azure Firewall provides Layer 7 filtering through application rules, allowing you to restrict traffic between tiers to specific FQDNs rather than just IP addresses, which is essential for PCI DSS compliance. Additionally, enabling diagnostic logs and sending them to a Log Analytics workspace captures all denied traffic for auditing, meeting the logging requirement efficiently. On the Microsoft Cybersecurity Architect exam, this scenario tests your ability to choose between Azure Firewall, NSGs, Application Gateway, and NVAs for inter-tier segmentation; a common trap is selecting NSGs, which lack L7 filtering and comprehensive denied-traffic logging. Remember the memory tip: “Firewall for FQDNs, NSGs for IPs” — when compliance demands application-layer control and audit logs, Azure Firewall is the single, cost-effective solution.
SC-100 Design security solutions for infrastructure Practice Question
This SC-100 practice question tests your understanding of design security solutions for infrastructure. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
You are designing a network security solution for a multi-tier application in Azure that must meet PCI DSS compliance. You need to restrict traffic between tiers to only necessary ports and protocols. You also need to log all denied traffic for auditing. What is the most efficient design?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Deploy Azure Firewall in a hub virtual network. Use application rules to allow specific FQDNs between tiers. Enable diagnostic logs and send them to a Log Analytics workspace.
Option A is correct because Azure Firewall with application rules provides L7 filtering, and diagnostic logs capture denied traffic. Option B is wrong because NSGs lack L7 filtering and logging of denied traffic is not as comprehensive. Option C is wrong because Application Gateway is a load balancer, not a firewall for inter-tier traffic. Option D is wrong because NVAs add complexity and cost without clear benefit over Azure Firewall for this use case.
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.
- ✗
Deploy Azure Application Gateway in front of each tier. Use WAF policies to filter traffic.
Why it's wrong here
Application Gateway is for inbound traffic, not inter-tier.
- ✗
Use network security groups (NSGs) on each subnet to allow/deny traffic by IP and port. Enable NSG flow logs for auditing.
Why it's wrong here
NSGs do not support L7 filtering and flow logs don't capture denied traffic.
- ✓
Deploy Azure Firewall in a hub virtual network. Use application rules to allow specific FQDNs between tiers. Enable diagnostic logs and send them to a Log Analytics workspace.
Why this is correct
Provides L7 filtering and logging.
Related concept
CIDR notation defines the prefix length.
- ✗
Deploy a third-party NVA (e.g., Palo Alto) in the hub. Configure inter-tier routing through the NVA.
Why it's wrong here
NVAs are valid but add complexity and cost; Azure Firewall is simpler.
Common exam traps
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
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 SC-100 subnetting questions on CIDR, address ranges, and subnet selection.
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Design security solutions for infrastructure — study guide chapter
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FAQ
Questions learners often ask
What does this SC-100 question test?
Design security solutions for infrastructure — This question tests Design security solutions for infrastructure — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Deploy Azure Firewall in a hub virtual network. Use application rules to allow specific FQDNs between tiers. Enable diagnostic logs and send them to a Log Analytics workspace. — Option A is correct because Azure Firewall with application rules provides L7 filtering, and diagnostic logs capture denied traffic. Option B is wrong because NSGs lack L7 filtering and logging of denied traffic is not as comprehensive. Option C is wrong because Application Gateway is a load balancer, not a firewall for inter-tier traffic. Option D is wrong because NVAs add complexity and cost without clear benefit over Azure Firewall for this use case.
What should I do if I get this SC-100 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 SC-100 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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Last reviewed: Jun 21, 2026
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