Question 34 of 1,621
ANS-C01 Network Implementation Practice Question
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?
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 network ACL for the private subnets is stateless and is blocking the return traffic from the instances to the ALB
The most likely cause is that the network ACL for the private subnets is stateless. While the inbound rule allows traffic on port 80 from the public subnet CIDR (which includes the ALB's IPs), the outbound rule does not allow the return traffic from the instances to the ALB. This return traffic uses ephemeral ports and must be explicitly allowed in the outbound rules of the private subnet NACL. Without this, the ALB cannot receive responses from the instances, causing health checks to fail and resulting in 502 errors. Option A is incorrect because the inbound NACL rule does allow traffic from the ALB's subnet. Option B is incorrect because the security group allows inbound traffic from the ALB's security group. Option D is incorrect because the target group port is correctly set to 80.
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
Why it's wrong here
It allows from the public subnet CIDR.
- ✗
The security group for the instances does not allow inbound traffic from the ALB
Why it's wrong here
It does allow from the ALB's security group.
- ✓
The network ACL for the private subnets is stateless and is blocking the return traffic from the instances to the ALB
Why this is correct
NACLs are stateless; outbound traffic from the private subnet to the ALB must be explicitly allowed.
- ✗
The target group is configured with the wrong port
Why it's wrong here
Port 80 is typical and the security group allows it.
Visual reference
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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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