The answer is that the stateful rule group is not configured to allow the traffic. Even when the stateless rule group is set to forward traffic, the stateful rule group acts as a second gate—if it lacks an explicit pass rule for the allowed traffic, it will silently drop packets by default. This is a critical nuance in AWS Network Firewall behavior: stateless rules handle initial forwarding decisions, but stateful rules inspect session context and require an explicit allow action for the traffic to proceed. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept frequently appears in questions about Transit Gateway inspection architectures, where a centralized inspection VPC uses Network Firewall to filter inter-VPC traffic. A common trap is assuming that stateless forwarding alone is sufficient, forgetting that stateful rules default to drop. Remember the two-gate principle: stateless forwards, stateful allows—if either gate is locked, traffic drops.
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 an AWS Network Firewall in a centralized inspection VPC to inspect traffic between VPCs connected to an AWS Transit Gateway. The architecture uses Transit Gateway route tables to send inter-VPC traffic through the inspection VPC. The Network Firewall is configured with stateful and stateless rule groups. After deployment, the security team notices that traffic from VPC A to VPC B is being dropped. Other traffic flows correctly. What is the MOST likely cause of this issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The stateless rule group is set to forward traffic, but the stateful rule group is not configured to allow the traffic.
Option B is correct because the stateful rule group must have a pass rule for the traffic. Option A is incorrect because stateless rules forward traffic but stateful rules can drop. Option C is incorrect because route tables in VPC A do not affect firewall. Option D is incorrect because Transit Gateway does not have 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.
✗
The route table for VPC A's subnets does not have a route to the inspection VPC.
Why it's wrong here
Route tables direct traffic to Transit Gateway, not to inspection VPC directly.
✓
The stateless rule group is set to forward traffic, but the stateful rule group is not configured to allow the traffic.
Why this is correct
Stateful rules process first and can drop traffic even if stateless allows.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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: The stateless rule group is set to forward traffic, but the stateful rule group is not configured to allow the traffic. — Option B is correct because the stateful rule group must have a pass rule for the traffic. Option A is incorrect because stateless rules forward traffic but stateful rules can drop. Option C is incorrect because route tables in VPC A do not affect firewall. Option D is incorrect because Transit Gateway does not have security groups.
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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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