The correct answer is to associate an IPv6 CIDR block with the public subnet where the ALB is deployed and add a route for ::/0 to the internet gateway. This is required because an internet-facing ALB operates at the subnet level, not the instance level; it inherits the subnet’s IPv6 CIDR to listen for and route IPv6 traffic. The ALB itself does not need a separate IPv6 address, as it uses the subnet’s IP allocation to handle native IPv6 client connections. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of how dual-stack networking works with load balancers, specifically that the subnet, not the ALB, must be IPv6-enabled. A common trap is confusing the ALB’s requirement with an instance’s need for an IPv6 address, or thinking an Egress-Only Internet Gateway applies to public subnets. Memory tip: “Subnet gets the CIDR, ALB gets the traffic—route ::/0 to the IGW for public IPv6 reach.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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. They need to add IPv6 support for their internet-facing Application Load Balancer. The VPC is already associated with an IPv6 CIDR block. What additional configuration is required? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add a route in the subnet's route table for ::/0 to the Internet Gateway.
Options A and C are correct. For an internet-facing ALB to serve IPv6 clients, the subnet must have an IPv6 CIDR and the route table must have a route for ::/0 to the internet gateway. Option B is wrong because the ALB itself does not require an IPv6 address; it uses the subnet's IPv6 CIDR. Option D is wrong because Egress-Only Internet Gateway is for outbound-only IPv6 traffic from private subnets. Option E is wrong because a NAT64/DNS64 is not needed; ALB can handle IPv6 natively.
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.
✗
Configure NAT64 and DNS64 on the VPC.
Why it's wrong here
NAT64/DNS64 is for translating IPv6 to IPv4, not needed for ALB IPv6 support.
✓
Add a route in the subnet's route table for ::/0 to the Internet Gateway.
Why this is correct
IPv6 traffic needs a route to the internet gateway.
Assign an IPv6 address to the ALB's network interface.
Why it's wrong here
ALB automatically gets an IPv6 address from the subnet's IPv6 CIDR; no manual assignment needed.
✗
Create an Egress-Only Internet Gateway for the VPC.
Why it's wrong here
Egress-Only Internet Gateway is for instances in private subnets to initiate outbound IPv6 traffic.
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.
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Add a route in the subnet's route table for ::/0 to the Internet Gateway. — Options A and C are correct. For an internet-facing ALB to serve IPv6 clients, the subnet must have an IPv6 CIDR and the route table must have a route for ::/0 to the internet gateway. Option B is wrong because the ALB itself does not require an IPv6 address; it uses the subnet's IPv6 CIDR. Option D is wrong because Egress-Only Internet Gateway is for outbound-only IPv6 traffic from private subnets. Option E is wrong because a NAT64/DNS64 is not needed; ALB can handle IPv6 natively.
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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