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Question 1,214 of 1,705

Network Implementation →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the target group’s IPv4-only configuration is not the root cause; instead, the private subnet lacks an IPv6 route to reach the IPv4-only targets. While an ALB with dual-stack IP address type can accept IPv6 traffic from the internet and terminate it, forwarding the request to IPv4 targets requires a translation mechanism—such as a NAT64 gateway or an egress-only internet gateway—and a corresponding IPv6 route in the private subnet’s route table. Without that route, the ALB’s attempt to connect to the IPv4 targets over IPv6 fails, even though the security group permits inbound IPv6 traffic. This scenario tests your understanding of how ALB dual-stack routing interacts with target group IP address types, a common trap on the AWS Certified Advanced Networking Specialty ANS-C01 exam where candidates mistakenly blame the ALB’s IP addressing or security groups. The key insight is that dual-stack ALBs can translate between protocols, but the network path must support IPv6 routing to the targets. Memory tip: “Dual-stack ALB can speak both, but the road to the target must be paved for IPv6.”

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 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?

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.

Question 1hardmultiple choice

Study the full IPv6 explanation →

A
The public subnet does not have an associated IPv6 CIDR.
Why wrong: ALB can use IPv6 even if subnet has IPv4 only? Actually, ALB needs IPv6 subnet.
B
The target group is configured with IP address type IPv4, but the ALB must use IPv6 to communicate with the targets.
ALB can communicate with IPv4 targets from IPv6 clients, but the issue might be that the ALB's security group or target group health checks fail. Actually, the correct answer is that the target group type is IPv4, which is fine. The real issue is that the private subnet needs an IPv6 route to the NAT64? Let's reconsider.
C
The ALB is configured as IPv4-only instead of dual-stack.
Why wrong: Stem says ALB is dual-stack.
D
The private subnet's route table does not have an IPv6 route to the NAT gateway or egress-only internet gateway.
Why wrong: Not needed for ALB to targets.

Full breakdown with real-world context →

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 target group is configured with IP address type IPv4, but the ALB must use IPv6 to communicate with the targets.

Option C is correct because an IPv4-only target group cannot receive IPv6 traffic; the ALB would attempt to connect to targets via IPv4, but the ALB can still accept IPv6 and translate. However, the issue is that the route table for the private subnet does not have an IPv6 route to the NAT64 or egress-only internet gateway? Actually, the most likely cause is that the private subnet lacks a route for IPv6 traffic from the ALB to reach the IPv4 targets. Option A is incorrect because the ALB can terminate IPv6 and forward to IPv4. Option B is incorrect because the ALB does not need an IPv6 address. Option D is incorrect because the ALB does not need an EIP.

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 public subnet does not have an associated IPv6 CIDR.
Why it's wrong here
ALB can use IPv6 even if subnet has IPv4 only? Actually, ALB needs IPv6 subnet.
✓
The target group is configured with IP address type IPv4, but the ALB must use IPv6 to communicate with the targets.
Why this is correct
ALB can communicate with IPv4 targets from IPv6 clients, but the issue might be that the ALB's security group or target group health checks fail. Actually, the correct answer is that the target group type is IPv4, which is fine. The real issue is that the private subnet needs an IPv6 route to the NAT64? Let's reconsider.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The ALB is configured as IPv4-only instead of dual-stack.
Why it's wrong here
Stem says ALB is dual-stack.
✗
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
Not needed for ALB to targets.

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.

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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 target group is configured with IP address type IPv4, but the ALB must use IPv6 to communicate with the targets. — Option C is correct because an IPv4-only target group cannot receive IPv6 traffic; the ALB would attempt to connect to targets via IPv4, but the ALB can still accept IPv6 and translate. However, the issue is that the route table for the private subnet does not have an IPv6 route to the NAT64 or egress-only internet gateway? Actually, the most likely cause is that the private subnet lacks a route for IPv6 traffic from the ALB to reach the IPv4 targets. Option A is incorrect because the ALB can terminate IPv6 and forward to IPv4. Option B is incorrect because the ALB does not need an IPv6 address. Option D is incorrect because the ALB does not need an EIP.

What should I do if I get this ANS-C01 question wrong?

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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