Question 1,214 of 1,621
ANS-C01 Network Implementation Practice Question
A company has a VPC with an IPv4 CIDR of 10.0.0.0/16 and an IPv6 CIDR of 2001:db8:1234::/56. The company hosts a web application on IPv4-only EC2 instances in a private subnet. The application must be accessible from the internet via IPv6. The company has an internet-facing Application Load Balancer (ALB) with dual-stack IP address type. The ALB is in a public subnet. The target group is configured with IP address type IPv4. Users report that they can access the application via IPv4 but not via IPv6. The ALB security group allows inbound HTTP/HTTPS from ::/0. What is the MOST likely cause?
⚠ Common exam trap
Candidates often assume that a dual-stack ALB automatically gets an IPv6 address, but it also requires the subnet to have an IPv6 CIDR.
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 public subnet does not have an associated IPv6 CIDR.
The most likely cause is that the public subnet does not have an associated IPv6 CIDR. For an ALB to accept IPv6 traffic, it must have an IPv6 address assigned to its network interface. This requires the public subnet to have an IPv6 CIDR block attached. Even though the ALB is configured as dual-stack, without an IPv6 CIDR on the subnet, the ALB cannot obtain an IPv6 address and therefore cannot receive IPv6 traffic. Option B is incorrect because an ALB can forward IPv6 client traffic to IPv4 targets using IPv4 communication; the target group IP address type does not need to be IPv6. Option C is false as the ALB is dual-stack. Option D is irrelevant because the private subnet's route table does not affect ALB-to-target communication.
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 public subnet does not have an associated IPv6 CIDR.
Why this is correct
Correct. Without an IPv6 CIDR on the public subnet, the ALB cannot obtain an IPv6 address, so it cannot receive IPv6 traffic despite being configured as dual-stack.
- ✗
The target group is configured with IP address type IPv4, but the ALB must use IPv6 to communicate with the targets.
Why it's wrong here
Incorrect. An ALB can forward IPv6 client traffic to IPv4 targets using IPv4 communication. The target group IP address type does not need to be IPv6 for the ALB to handle IPv6 traffic.
- ✗
The ALB is configured as IPv4-only instead of dual-stack.
Why it's wrong here
Incorrect. The ALB is configured as dual-stack, so it can accept both IPv4 and IPv6 traffic if the subnet has an IPv6 CIDR.
- ✗
The private subnet's route table does not have an IPv6 route to the NAT gateway or egress-only internet gateway.
Why it's wrong here
Incorrect. The private subnet's route table does not affect ALB-to-target communication; the ALB sends traffic to targets using private IPs within the VPC, not through NAT or egress-only internet gateway.
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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