The answer is that the security group for the EC2 instances does not allow inbound traffic from the ALB’s security group on the health check port. This is because the Application Load Balancer performs health checks against the target instances using a specific port and protocol, and if the EC2 security group does not explicitly permit this traffic from the ALB’s security group, the health checks fail, marking the targets as unhealthy even if the application is running. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how security group chaining works with ALB health checks, a common trap where engineers focus on internet-facing rules or NAT gateways but overlook the critical inbound rule for the health check port. Remember that the ALB’s security group acts as a source in the EC2 security group rule, not the ALB’s IP range, and the health check traffic must be allowed on the listener’s port or a dedicated health check port. A useful memory tip is “health check, check the SG chain” — always verify that the target security group explicitly references the ALB security group on the health check port.
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 a web application on Amazon EC2 instances behind an Application Load Balancer (ALB). The ALB is internet-facing and uses a public subnet. The EC2 instances are in private subnets. The application needs to be accessible from the internet. The security group for the ALB allows inbound HTTP and HTTPS from 0.0.0.0/0. The security group for the EC2 instances allows inbound traffic from the ALB's security group. The route tables for the private subnets have a default route to a NAT gateway. Users report that they cannot access the application. The ALB target group shows the instances 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The security group for the EC2 instances does not allow inbound traffic from the ALB's security group on the health check port.
Option D is correct because the security group for EC2 instances must allow health check traffic from the ALB. Option A is incorrect because ALB is in public subnet. Option B is incorrect because ALB does not need internet gateway; it's already public. Option C is incorrect because NAT gateway is for outbound traffic.
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 security group for the EC2 instances does not allow inbound traffic from the ALB's security group on the health check port.
Why this is correct
Health checks fail if security group blocks traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The ALB is deployed in a private subnet instead of a public subnet.
Why it's wrong here
Stem says ALB is in public subnet.
✗
The ALB does not have an associated internet gateway route.
Why it's wrong here
ALB uses its own internet-facing nature; no route needed.
✗
The NAT gateway is not correctly routing traffic from the instances to the internet.
Why it's wrong here
Not relevant for inbound traffic.
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 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 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 security group for the EC2 instances does not allow inbound traffic from the ALB's security group on the health check port. — Option D is correct because the security group for EC2 instances must allow health check traffic from the ALB. Option A is incorrect because ALB is in public subnet. Option B is incorrect because ALB does not need internet gateway; it's already public. Option C is incorrect because NAT gateway is for outbound traffic.
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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