The correct set of steps is to associate an Amazon-provided IPv6 CIDR block to the VPC, add a route for ::/0 to the Internet Gateway, and allocate IPv6 addresses to the subnets. This works because IPv6 in AWS requires a globally unique, Amazon-provided /56 CIDR block for the VPC, and the Internet Gateway is a dual-stack resource that can route both IPv4 and IPv6 traffic, so adding a default IPv6 route (::/0) to the existing IGW enables public inbound and outbound connectivity. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of dual-stack VPC design and the distinction between an Internet Gateway (which supports inbound IPv6) and an Egress-Only Internet Gateway (which is outbound-only). A common trap is confusing the two gateway types or assuming you can assign a custom IPv6 block without a separate request. Memory tip: think “IGW handles both stacks, EIGW is IPv6-only outbound.”
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 needs to add IPv6 support for its public-facing web application. The application must be accessible via both IPv4 and IPv6. The VPC already has an Internet Gateway attached. What is the correct set of steps to enable IPv6?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Associate an Amazon-provided IPv6 CIDR block to the VPC, add a route for ::/0 to the Internet Gateway, and allocate IPv6 addresses to the subnets.
Option D is correct because to enable IPv6, you must associate an Amazon-provided IPv6 CIDR block with the VPC, update the route table to include a route for ::/0 to the Internet Gateway, and ensure the subnet has an IPv6 CIDR. Option A is wrong because you cannot assign a /64 block to a VPC. Option B is wrong because Egress-Only Internet Gateway is for outbound-only IPv6, not inbound. Option C is wrong because you cannot assign a custom IPv6 block without requesting it.
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.
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Assign a custom IPv6 CIDR block from your own range to the VPC and add a route to the Internet Gateway.
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: Associate an Amazon-provided IPv6 CIDR block to the VPC, add a route for ::/0 to the Internet Gateway, and allocate IPv6 addresses to the subnets. — Option D is correct because to enable IPv6, you must associate an Amazon-provided IPv6 CIDR block with the VPC, update the route table to include a route for ::/0 to the Internet Gateway, and ensure the subnet has an IPv6 CIDR. Option A is wrong because you cannot assign a /64 block to a VPC. Option B is wrong because Egress-Only Internet Gateway is for outbound-only IPv6, not inbound. Option C is wrong because you cannot assign a custom IPv6 block without requesting it.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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