The best approach is to create a firewall VPC with the appliance and use Transit Gateway route tables to direct traffic through it. This works because the Transit Gateway acts as a central hub, and by attaching a dedicated VPC hosting the third-party firewall, you can manipulate the route tables to force all inter-VPC traffic to pass through that inspection VPC before reaching its destination. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of centralized egress and inspection architectures, often appearing as a question about how to inspect traffic between VPCs using a third-party firewall transit gateway without introducing asymmetric routing. A common trap is assuming you can attach the firewall directly to a VPC subnet or use a Network Load Balancer, but the correct design always relies on route table propagation and static routes in the Transit Gateway to steer traffic. Remember the mnemonic: “Route through the gateway, inspect in the middle” — the firewall VPC is the middleman, and the route tables are the traffic cops.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 has an AWS Transit Gateway with multiple VPC attachments. They need to inspect traffic between VPCs using a third-party firewall appliance. What is the best approach?
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
✓
Create a firewall VPC with the appliance and use Transit Gateway route tables to direct traffic through it
Placing a firewall appliance in a dedicated VPC and routing traffic through it via Transit Gateway route tables allows inspection.
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.
✓
Create a firewall VPC with the appliance and use Transit Gateway route tables to direct traffic through it
Why this is correct
This allows centralized inspection via routing.
Clue confirmation
The clue word "best" in the question point toward this answer.
Security groups cannot be applied across VPC peering or Transit Gateway.
✗
Use AWS Network Firewall directly in each VPC
Why it's wrong here
While possible, it is less centralized and more complex than a single inspection VPC.
✗
Use network ACLs on each VPC subnet
Why it's wrong here
Network ACLs are stateless and do not provide deep packet inspection.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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: Create a firewall VPC with the appliance and use Transit Gateway route tables to direct traffic through it — Placing a firewall appliance in a dedicated VPC and routing traffic through it via Transit Gateway route tables allows inspection.
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.
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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Question Discussion
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