The answer is that the network ACL for the private subnets is blocking the return traffic due to its stateless nature, causing the ALB health check fail. While the NACL correctly allows inbound traffic on port 80 from the public subnet CIDR (10.0.1.0/24), which includes the ALB’s private IPs, it is stateless and therefore does not automatically permit the outbound response traffic from the EC2 instances back to the ALB. This missing outbound rule for ephemeral ports (typically 1024-65535) means the health check responses are dropped, resulting in 502 errors. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the fundamental difference between stateful security groups and stateless NACLs—a common trap where candidates focus only on inbound rules. Remember the mnemonic: NACLs are “no automatic return,” so always check both inbound and outbound directions for health check traffic.
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 has a VPC with a CIDR of 10.0.0.0/16. They have an Application Load Balancer (ALB) in public subnets and EC2 instances in private subnets. The ALB is configured to route traffic to the instances. Users can reach the ALB, but the ALB returns 502 Bad Gateway errors. The target group health checks are failing. The instances are running a web server on port 80. The security group for the instances allows inbound traffic from the ALB's security group on port 80. The network ACL for the private subnets allows inbound traffic on port 80 from the public subnet CIDR (10.0.1.0/24). What is the most likely cause of the health check failures?
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 network ACL for the private subnets is stateless and is blocking the return traffic from the instances to the ALB
Option B is correct because the health checks originate from the ALB's private IPs, which are in the public subnet CIDR 10.0.1.0/24, but the network ACL must allow inbound traffic from the ALB's security group source? Actually, the NACL allows from the public subnet CIDR, which should include the ALB's IPs. However, the NACL is stateless and must also allow outbound traffic from the instance's subnet to the ALB. The issue is likely that the NACL is blocking the return traffic. Option A is wrong because the security group allows the traffic. Option C is wrong because the NACL allows inbound on port 80 from the public subnet. Option D is wrong because the target group port is correct.
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 network ACL for the private subnets does not allow inbound traffic on port 80 from the ALB's subnet
The target group is configured with the wrong port
Why it's wrong here
Port 80 is typical and the security group allows it.
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: The network ACL for the private subnets is stateless and is blocking the return traffic from the instances to the ALB — Option B is correct because the health checks originate from the ALB's private IPs, which are in the public subnet CIDR 10.0.1.0/24, but the network ACL must allow inbound traffic from the ALB's security group source? Actually, the NACL allows from the public subnet CIDR, which should include the ALB's IPs. However, the NACL is stateless and must also allow outbound traffic from the instance's subnet to the ALB. The issue is likely that the NACL is blocking the return traffic. Option A is wrong because the security group allows the traffic. Option C is wrong because the NACL allows inbound on port 80 from the public subnet. Option D is wrong because the target group port is correct.
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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