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Network Implementation →hardMultiple ChoiceObjective-mapped

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.

A
The public subnet does not have an associated IPv6 CIDR.
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.
B
The target group is configured with IP address type IPv4, but the ALB must use IPv6 to communicate with the targets.
Why wrong: 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.
C
The ALB is configured as IPv4-only instead of dual-stack.
Why wrong: Incorrect. The ALB is configured as dual-stack, so it can accept both IPv4 and IPv6 traffic if the subnet has an IPv6 CIDR.
D
The private subnet's route table does not have an IPv6 route to the NAT gateway or egress-only internet gateway.
Why wrong: 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.

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 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.

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 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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
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.

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.

Visual reference

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 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.

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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