- A
The network ACL for the private subnets is blocking inbound traffic from the ALB.
Why wrong: Network ACLs are stateless and must allow both inbound and outbound; but the more likely issue is health check configuration.
- B
The security group for the web servers does not allow inbound traffic from the internet.
Why wrong: Traffic from the ALB originates from its private IPs within the VPC, not directly from the internet.
- C
The ALB is deployed in private subnets and cannot reach the internet.
Why wrong: The scenario states ALB is in public subnets.
- D
The health check is configured to use the public IP address of the targets, but the targets are in private subnets without public IPs.
Health checks must target the private IP of the instances; using public IPs will fail.
Quick Answer
The answer is that the health check is configured to use the public IP address of the targets, but the instances reside in private subnets without public IPs. This is the most likely cause because an ALB health check must reach the target over a routable IP; if the health check is set to the public IP, the ALB will attempt to connect to an address the private instance does not own, causing the target to appear unhealthy. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how ALB health checks interact with network routing and subnet architecture—a common trap is assuming the ALB can magically reach a private instance via a public IP. Remember that private instances only respond to traffic from within the VPC, so the health check must target their private IP. Memory tip: “Public check, private deck—target stays a wreck.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 deployed a web application behind an Application Load Balancer (ALB) in a VPC with public and private subnets. The ALB is in public subnets, and the web servers are in private subnets. Clients report intermittent connection errors. Investigation shows that the ALB is marking targets as unhealthy. What is the MOST likely cause?
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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The health check is configured to use the public IP address of the targets, but the targets are in private subnets without public IPs.
If the ALB health checks are configured to use the public IP of the targets, the private subnet instances will not respond because they do not have public IPs. Option A is wrong because security groups should allow traffic from the ALB. Option B is wrong because the ALB is in public subnets. Option D is wrong because security groups should be on the instance side.
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 is blocking inbound traffic from the ALB.
Why it's wrong here
Network ACLs are stateless and must allow both inbound and outbound; but the more likely issue is health check configuration.
- ✗
The security group for the web servers does not allow inbound traffic from the internet.
Why it's wrong here
Traffic from the ALB originates from its private IPs within the VPC, not directly from the internet.
- ✗
The ALB is deployed in private subnets and cannot reach the internet.
Why it's wrong here
The scenario states ALB is in public subnets.
- ✓
The health check is configured to use the public IP address of the targets, but the targets are in private subnets without public IPs.
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.
Trap categories for this question
Scenario analysis trap
The scenario states ALB is in public subnets.
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.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
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 health check is configured to use the public IP address of the targets, but the targets are in private subnets without public IPs. — If the ALB health checks are configured to use the public IP of the targets, the private subnet instances will not respond because they do not have public IPs. Option A is wrong because security groups should allow traffic from the ALB. Option B is wrong because the ALB is in public subnets. Option D is wrong because security groups should be on the instance side.
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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Last reviewed: Jun 20, 2026
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.
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