The answer is to implement network micro-segmentation using Azure Firewall and NSGs to isolate the legacy PLCs from the corporate network. This approach is correct because unpatched legacy PLCs cannot run security agents or support modern encryption like TLS, so the only effective defense is to restrict lateral movement by placing them in a separate network segment with tightly controlled traffic flows. On the Microsoft Cybersecurity Architect exam, this scenario tests your understanding of defense-in-depth for operational technology (OT) environments, where legacy devices must be isolated rather than hardened. A common trap is assuming you can apply host-based controls or VPNs, but those either require agent support or expand the attack surface. Remember the memory tip: “Legacy means isolate, not integrate”—when a device cannot be patched, your only option is to contain it with network boundaries.
SC-100 Design security solutions for infrastructure Practice Question
This SC-100 practice question tests your understanding of design security solutions for infrastructure. 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.
You are designing a secure access strategy for a manufacturing plant using Azure IoT Hub and Azure Defender for IoT. The plant has unpatched legacy PLCs that cannot be updated. What is the best approach to prevent these devices from being compromised and used as an entry point into the corporate network?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Implement network micro-segmentation using Azure Firewall and NSGs to isolate the PLCs from the corporate network.
Option D is correct because micro-segmentation with network security groups (NSGs) and Azure Firewall isolates the legacy devices, limiting lateral movement. Option A is wrong because unpatched devices cannot be hardened with Defender for IoT agents. Option B is wrong because TLS is often not supported on legacy PLCs. Option C is wrong because VPN would expose the devices to more traffic.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use Azure VPN Gateway to connect PLCs to the virtual network.
Why it's wrong here
VPN does not compensate for unpatched vulnerabilities.
✓
Implement network micro-segmentation using Azure Firewall and NSGs to isolate the PLCs from the corporate network.
Why this is correct
Isolation limits the blast radius if a device is compromised.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
Install the Microsoft Defender for IoT micro-agent on each PLC.
Why it's wrong here
Legacy PLCs may not support agent installation and cannot be updated.
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this SC-100 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SC-100 NAT questions on configuration and troubleshooting.
Design security solutions for infrastructure — This question tests Design security solutions for infrastructure — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Implement network micro-segmentation using Azure Firewall and NSGs to isolate the PLCs from the corporate network. — Option D is correct because micro-segmentation with network security groups (NSGs) and Azure Firewall isolates the legacy devices, limiting lateral movement. Option A is wrong because unpatched devices cannot be hardened with Defender for IoT agents. Option B is wrong because TLS is often not supported on legacy PLCs. Option C is wrong because VPN would expose the devices to more traffic.
What should I do if I get this SC-100 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SC-100 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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