The answer is that the firewall endpoint resides in a different Availability Zone than the EC2 instance, causing a cross-AZ routing failure. AWS Network Firewall endpoints are zonal resources, meaning traffic must be routed to an endpoint within the same Availability Zone as the source instance to function correctly; routing traffic across zones introduces asymmetric routing and potential packet drops, as the firewall’s stateful engine expects return traffic to traverse the same endpoint. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of zonal service architecture and the importance of aligning route table entries with the instance’s Availability Zone. A common trap is assuming a single firewall endpoint can serve all subnets, but the exam emphasizes that each AZ requires its own endpoint for proper traffic inspection. Remember the mnemonic: “Same zone, same stone” — always route to the firewall endpoint in the instance’s own Availability Zone.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 has a VPC with multiple subnets. They deploy a Network Firewall to inspect traffic. The firewall is configured with a stateful rule that allows outbound HTTP traffic to any destination. However, traffic from an EC2 instance in a private subnet to an external web server fails. The route table for the private subnet has a default route pointing to the firewall endpoint. What is the likely issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The firewall endpoint is in a different Availability Zone than the instance.
Option D is correct because the firewall endpoint is in a different Availability Zone; traffic must be routed to the firewall endpoint in the same AZ to avoid cross-AZ charges and potential routing issues. Option A is wrong because the stateful rule allows HTTP outbound. Option B is wrong because a NATgateway is not required if using firewall. Option C is wrong because the security group allows outbound HTTP.
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 stateful rule order is incorrect; a default deny rule is blocking traffic.
Why it's wrong here
The stateful rule allows HTTP, so traffic should be permitted.
✗
The EC2 instance security group does not allow outbound HTTP.
Why it's wrong here
The question states security group allows all outbound traffic.
✓
The firewall endpoint is in a different Availability Zone than the instance.
Why this is correct
Traffic must be sent to the firewall endpoint in the same AZ; otherwise routing may fail.
The subnet does not have a route to a NAT gateway.
Why it's wrong here
Network Firewall can provide outbound internet access without a NATgateway.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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 Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The firewall endpoint is in a different Availability Zone than the instance. — Option D is correct because the firewall endpoint is in a different Availability Zone; traffic must be routed to the firewall endpoint in the same AZ to avoid cross-AZ charges and potential routing issues. Option A is wrong because the stateful rule allows HTTP outbound. Option B is wrong because a NAT gateway is not required if using firewall. Option C is wrong because the security group allows outbound HTTP.
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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This ANS-C01 practice question is part of Courseiva's free Amazon Web Services 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 ANS-C01 exam.
Question Discussion
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