- A
Move each application tier to a separate VPC and use VPC peering with strict routing policies
Why wrong: This adds unnecessary complexity and cost; VPC peering does not provide micro-segmentation within a tier.
- B
Place all VMs in the same subnet and configure security groups that only allow necessary inter-VM traffic (e.g., web-to-db), with a default deny rule
Security groups are stateful and evaluated at the instance level; they allow fine-grained control with default deny.
- C
Place all VMs in the same subnet and configure network ACLs with explicit allow rules for required traffic and a deny-all rule for other traffic
Why wrong: Network ACLs are stateless, requiring both inbound and outbound rules, which adds complexity; also they apply to the entire subnet, not individual VMs.
- D
Keep all VMs in the same subnet but deploy a third-party next-generation firewall as a virtual appliance to inspect and filter inter-VM traffic
Why wrong: While possible, it introduces additional cost and overhead; native security groups are simpler and sufficient.
CAS-004 Security Architecture Practice Question
This CAS-004 practice question tests your understanding of security architecture. 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.
An organization has recently migrated its on-premises data center to a public cloud. The security team notices that several virtual machines (VMs) in the same subnet are communicating with each other without any restrictions. The company policy requires that only specific application traffic (e.g., database queries from web servers) be allowed between VMs, and all other inter-VM traffic must be blocked to comply with a zero-trust model. The cloud provider offers native security group and network ACL features. The architect must design a solution that enforces the policy with minimal administrative overhead and supports future expansion.
Which of the following is the BEST course of action?
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 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
Place all VMs in the same subnet and configure security groups that only allow necessary inter-VM traffic (e.g., web-to-db), with a default deny rule
Using security groups (instance-level firewalls) allows per-VM permit rules and denies by default, meeting zero-trust requirements. Option B is wrong because network ACLs are stateless and require rules for both directions, increasing complexity. Option C is wrong because moving to different VPCs breaks application connectivity unnecessarily. Option D is wrong because a third-party firewall is an additional cost and complexity without clear benefit over native security groups.
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.
- ✗
Move each application tier to a separate VPC and use VPC peering with strict routing policies
Why it's wrong here
This adds unnecessary complexity and cost; VPC peering does not provide micro-segmentation within a tier.
- ✓
Place all VMs in the same subnet and configure security groups that only allow necessary inter-VM traffic (e.g., web-to-db), with a default deny rule
Why this is correct
Security groups are stateful and evaluated at the instance level; they allow fine-grained control with default deny.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
Place all VMs in the same subnet and configure network ACLs with explicit allow rules for required traffic and a deny-all rule for other traffic
Why it's wrong here
Network ACLs are stateless, requiring both inbound and outbound rules, which adds complexity; also they apply to the entire subnet, not individual VMs.
- ✗
Keep all VMs in the same subnet but deploy a third-party next-generation firewall as a virtual appliance to inspect and filter inter-VM traffic
Why it's wrong here
While possible, it introduces additional cost and overhead; native security groups are simpler and sufficient.
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 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 CAS-004 subnetting questions on CIDR, address ranges, and subnet selection.
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Security Architecture — study guide chapter
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FAQ
Questions learners often ask
What does this CAS-004 question test?
Security Architecture — This question tests Security Architecture — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Place all VMs in the same subnet and configure security groups that only allow necessary inter-VM traffic (e.g., web-to-db), with a default deny rule — Using security groups (instance-level firewalls) allows per-VM permit rules and denies by default, meeting zero-trust requirements. Option B is wrong because network ACLs are stateless and require rules for both directions, increasing complexity. Option C is wrong because moving to different VPCs breaks application connectivity unnecessarily. Option D is wrong because a third-party firewall is an additional cost and complexity without clear benefit over native security groups.
What should I do if I get this CAS-004 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 CAS-004 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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Last reviewed: Jun 24, 2026
This CAS-004 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CAS-004 exam.
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