The answer is to place each tier in a separate subnet and apply Network Security Groups (NSGs) with specific inbound and outbound rules. This approach is correct because NSGs provide stateful, granular traffic control, allowing you to define rules that permit web-to-app traffic on port 443 and app-to-database traffic on port 1433, while implicitly denying all other cross-tier communication—including web-to-database—through the default deny-all rule. On the Microsoft Cybersecurity Architect exam, this scenario tests your understanding of NSG subnet segmentation for three-tier applications, a core concept in designing secure network perimeters within Azure. A common trap is assuming Application Security Groups alone suffice without subnet isolation, but the most secure and efficient method combines subnet-level segmentation with NSG rules to enforce strict east-west traffic flows. Memory tip: think of the three tiers as a chain—web talks only to app, app only to DB—and NSGs as the bouncers at each subnet door, checking both source and destination.
SC-100 Design security for infrastructure Practice Question
This SC-100 practice question tests your understanding of design security 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.
A company deploys a three-tier application with web servers, application servers, and database servers in a VNet. They need to ensure that web servers can only communicate with application servers on port 443, and application servers can only communicate with database servers on port 1433. Web servers should not be able to communicate with database servers. What is the most secure and efficient way to implement this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Place each tier in a separate subnet, and create NSGs on each subnet with appropriate inbound and outbound rules.
Option A is correct because placing each tier in a separate subnet and applying Network Security Groups (NSGs) with specific inbound and outbound rules provides network segmentation and granular traffic control. NSGs are stateful, so you can define rules that allow web servers to initiate outbound traffic to application servers on port 443 and application servers to initiate outbound traffic to database servers on port 1433, while implicitly denying all other cross-tier communication (including web-to-database). This approach is both secure and efficient as it uses native Azure constructs without additional cost or complexity.
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.
✓
Place each tier in a separate subnet, and create NSGs on each subnet with appropriate inbound and outbound rules.
Why this is correct
This provides network segmentation and granular control.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use Azure Firewall to inspect all traffic between tiers.
Why it's wrong here
Unnecessary complexity and cost; NSGs are sufficient for this scenario.
✗
Use Application Security Groups (ASGs) to group VMs, but do not create NSGs.
Why it's wrong here
ASGs alone do not enforce traffic rules; NSGs are required.
✗
Place all tiers in the same subnet, and use a single NSG with rules to allow and deny traffic.
Why it's wrong here
Single subnet does not provide isolation; NSGs would be complex and error-prone.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often overlook that NSGs do not filter traffic between resources within the same subnet, leading them to choose Option D, which would fail to isolate the tiers.
Trap categories for this question
Scenario analysis trap
Unnecessary complexity and cost; NSGs are sufficient for this scenario.
Detailed technical explanation
How to think about this question
NSGs are stateful and evaluate rules in priority order; when you create an outbound rule allowing traffic from the web subnet to the app subnet on port 443, the return traffic is automatically allowed without an explicit inbound rule. For database access, the application servers must initiate outbound traffic to the database subnet on port 1433 (typically SQL Server), and the NSG on the database subnet must have an inbound rule allowing that traffic. A common subtlety is that NSGs can be applied at both the subnet and NIC level, with subnet-level rules evaluated first, so overlapping rules can cause unexpected denies if not carefully designed.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this SC-100 question in full detail.
Design security for infrastructure — This question tests Design security for infrastructure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Place each tier in a separate subnet, and create NSGs on each subnet with appropriate inbound and outbound rules. — Option A is correct because placing each tier in a separate subnet and applying Network Security Groups (NSGs) with specific inbound and outbound rules provides network segmentation and granular traffic control. NSGs are stateful, so you can define rules that allow web servers to initiate outbound traffic to application servers on port 443 and application servers to initiate outbound traffic to database servers on port 1433, while implicitly denying all other cross-tier communication (including web-to-database). This approach is both secure and efficient as it uses native Azure constructs without additional cost or complexity.
What should I do if I get this SC-100 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.
About these practice questions
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Variation 1. A financial services company is deploying a three-tier application on Azure. They need to ensure that the web tier can only communicate with the application tier, and the application tier can only communicate with the data tier. All tiers should use private IP addresses. What is the most secure way to implement this?
medium
✓ A.Deploy Azure Firewall and use application rules
B.Use Network Security Groups (NSGs) on each subnet
C.Use VNet peering with route tables
D.Use Azure Web Application Firewall (WAF)
Why A: Azure Firewall with application rules (FQDN-based) provides the most secure and granular control for east-west traffic between tiers. It can inspect and filter traffic at Layer 7 (application layer) using TLS inspection, ensuring only allowed application protocols (e.g., HTTPS) and specific FQDNs are permitted, while blocking all other traffic. This meets the requirement for private IP communication and enforces a zero-trust model between tiers.
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Question Discussion
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