The answer is an egress-only internet gateway (EIGW) for IPv6 outbound from private subnets. This is correct because an EIGW enables outbound-only IPv6 traffic from instances in private subnets without allowing inbound connections, which is exactly what the scenario requires—the VPC is not configured for IPv6, so you must add an IPv6 CIDR block, assign IPv6 addresses to the private subnets, and route ::/0 to the EIGW. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of dual-stack VPC design and the distinction between NAT gateways (IPv4 only) and EIGWs (IPv6 only). A common trap is confusing NAT64 with EIGW: NAT64 translates IPv6 to IPv4, but here the external service uses IPv6, so translation is unnecessary. Another trap is assuming an internet gateway alone works for private subnets—it does not, because private instances lack public IPv6 addresses. Memory tip: EIGW = “Egress Only” = “Exit Only” for IPv6, like a one-way door for private subnets.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 uses a VPC with multiple subnets in different Availability Zones. The VPC has a NAT Gateway in a public subnet of us-east-1a, and a second NAT Gateway in us-east-1b for high availability. Each private subnet in us-east-1a routes 0.0.0.0/0 to the NAT Gateway in us-east-1a, and private subnets in us-east-1b route to the NAT Gateway in us-east-1b. The company's EC2 instances in private subnets need to access an external service using IPv6. The VPC is not configured for IPv6. The network engineer needs to enable IPv6 connectivity for these instances. Which solution is the most cost-effective and scalable?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add an IPv6 CIDR block to the VPC, assign IPv6 addresses to private subnets, and add a route for ::/0 to an egress-only internet gateway.
Option C is correct because using an egress-only internet gateway (EIGW) provides IPv6 outbound connectivity for instances in private subnets when the VPC is dual-stack. Option A is wrong because NAT64 translates IPv6 to IPv4, but the service is IPv6, so not needed. Option B is wrong because adding IPv6 CIDR and using NAT64 is not necessary. Option D is wrong because an internet gateway alone does not work for private subnets; it requires a route and instances need public IPv6 addresses.
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.
✗
Add an IPv6 CIDR block to the VPC and configure a NAT64 gateway to translate IPv6 to IPv4.
Why it's wrong here
NAT64 is for IPv6-only clients accessing IPv4 services, not the other way.
✓
Add an IPv6 CIDR block to the VPC, assign IPv6 addresses to private subnets, and add a route for ::/0 to an egress-only internet gateway.
Why this is correct
Egress-only IGW allows outbound IPv6 traffic from private subnets.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
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 Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Add an IPv6 CIDR block to the VPC, assign IPv6 addresses to private subnets, and add a route for ::/0 to an egress-only internet gateway. — Option C is correct because using an egress-only internet gateway (EIGW) provides IPv6 outbound connectivity for instances in private subnets when the VPC is dual-stack. Option A is wrong because NAT64 translates IPv6 to IPv4, but the service is IPv6, so not needed. Option B is wrong because adding IPv6 CIDR and using NAT64 is not necessary. Option D is wrong because an internet gateway alone does not work for private subnets; it requires a route and instances need public IPv6 addresses.
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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