The answer is a restrictive network ACL on the private subnets blocking inbound traffic from the ALB’s subnets. This is the most likely cause because while the EC2 instance security group correctly allows traffic from the ALB’s security group, network ACLs are stateless and operate at the subnet level, meaning they can silently drop traffic even if the security group permits it. In a blue/green deployment, CodeDeploy launches new instances in private subnets, and the ALB’s health checks—originating from its IPs in public subnets—must traverse the private subnet’s network ACL; if that ACL does not explicitly allow inbound traffic from the ALB’s subnet CIDR on the health check port, the checks fail. On the DVA-C02 exam, this scenario tests your understanding of the layered security model: security groups are stateful and instance-level, while network ACLs are stateless and subnet-level—a common trap is assuming security group rules alone guarantee connectivity. Memory tip: think “ACL blocks subnet, SG allows instance”—if health checks fail despite correct SGs, check the subnet’s ACL first.
DVA-C02 Deployment Practice Question
This DVA-C02 practice question tests your understanding of deployment. 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 production environment running on AWS. The environment includes an Application Load Balancer (ALB) in front of an Auto Scaling group of EC2 instances. The application is deployed using AWS CodeDeploy with a blue/green deployment strategy. Recently, the deployment started failing because the new instances do not pass the health checks configured on the ALB. The health check path is '/health'. The developer has verified that the application starts correctly and responds to the health check on the new instances when accessed directly via the instance's private IP. However, the health checks from the ALB are failing. The security group for the ALB allows inbound traffic on port 80 from 0.0.0.0/0, and the security group for the EC2 instances allows inbound traffic on port 80 from the ALB's security group. The VPC has both public and private subnets. The Auto Scaling group launches instances in private subnets. The ALB is in public subnets. What is the MOST likely cause of the health check failure?
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 on the private subnets is blocking inbound traffic from the ALB's subnets.
Option A is correct because the ALB's health checks originate from its own IP addresses, which are in the public subnets. The instance security group must allow traffic from the ALB's security group, which it does. However, the issue might be that the instances have a restrictive network ACL on the private subnets that blocks inbound traffic from the ALB's subnet. Option B is wrong because the application works when accessed directly. Option C is wrong because the health check path is correct. Option D is wrong because the ALB is in public subnets and can reach private instances via NAT if needed.
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 application is not listening on port 80 on the new instances.
Why it's wrong here
The application works when accessed directly.
✗
The health check path is configured incorrectly in the ALB target group.
Why it's wrong here
It is '/health'.
✗
The ALB is not in the same VPC as the instances.
Why it's wrong here
They are in the same VPC.
✓
The network ACL on the private subnets is blocking inbound traffic from the ALB's subnets.
Why this is correct
Network ACLs are stateless and can block traffic even if security groups allow it.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 DVA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Deployment — This question tests Deployment — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The network ACL on the private subnets is blocking inbound traffic from the ALB's subnets. — Option A is correct because the ALB's health checks originate from its own IP addresses, which are in the public subnets. The instance security group must allow traffic from the ALB's security group, which it does. However, the issue might be that the instances have a restrictive network ACL on the private subnets that blocks inbound traffic from the ALB's subnet. Option B is wrong because the application works when accessed directly. Option C is wrong because the health check path is correct. Option D is wrong because the ALB is in public subnets and can reach private instances via NAT if needed.
What should I do if I get this DVA-C02 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 DVA-C02 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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