The correct answer is to deploy a centralized firewall appliance in a shared VPC and use Transit Gateway to route all inter-VPC traffic through it. This works because a stateful firewall can track connection state, allowing only web-to-app traffic while blocking any return or unsolicited connections from the app tier, and Transit Gateway route tables can be configured to force all inter-VPC traffic through that inspection VPC. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how to enforce unidirectional traffic between VPCs using Transit Gateway and firewall, often appearing as a trap where candidates mistakenly choose stateless NACLs or non-transitive VPC peering. The key insight is that security groups are stateful but cannot be applied across VPC boundaries, and NACLs lack state awareness. Remember the mnemonic: “Firewall for flow, Transit for transit” — the firewall controls direction, the Transit Gateway provides the path.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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.
A company is deploying a multi-tier application across multiple VPCs connected via AWS Transit Gateway. The web tier must be able to initiate connections to the app tier, but the app tier must not be able to initiate connections to the web tier. How can this be achieved?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Deploy a centralized firewall appliance in a shared VPC and use Transit Gateway to route all inter-VPC traffic through it.
Option D is correct because a stateful network firewall appliance in a central VPC can enforce unidirectional traffic rules. Transit Gateway route tables can direct traffic to the firewall VPC for inspection, and the firewall rules allow only web-to-app traffic. Option A is wrong because NACLs are stateless and cannot track connection state. Option B is wrong because VPC peering does not provide transitive routing. Option C is wrong because security groups are stateful but cannot be applied across VPCs.
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.
✗
Configure network ACLs on the web tier subnets to allow outbound traffic and deny inbound from app tier.
Why it's wrong here
NACLs are stateless; return traffic would be blocked.
✗
Establish VPC peering between the web and app VPCs and restrict routes.
Why it's wrong here
Peering is transitive only via Transit Gateway, and does not enforce unidirectional traffic.
✗
Use security groups across the VPCs by referencing each other's CIDR blocks.
Why it's wrong here
Security groups cannot reference other VPCs' groups without AWS PrivateLink.
✓
Deploy a centralized firewall appliance in a shared VPC and use Transit Gateway to route all inter-VPC traffic through it.
Why this is correct
Enables stateful inspection and unidirectional rules.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Deploy a centralized firewall appliance in a shared VPC and use Transit Gateway to route all inter-VPC traffic through it. — Option D is correct because a stateful network firewall appliance in a central VPC can enforce unidirectional traffic rules. Transit Gateway route tables can direct traffic to the firewall VPC for inspection, and the firewall rules allow only web-to-app traffic. Option A is wrong because NACLs are stateless and cannot track connection state. Option B is wrong because VPC peering does not provide transitive routing. Option C is wrong because security groups are stateful but cannot be applied across VPCs.
What should I do if I get this ANS-C01 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 ANS-C01 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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Question Discussion
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