The answer is that the instances do not have an IPv6 address assigned. An egress-only internet gateway enables outbound-only IPv6 traffic from a VPC, but it can only forward traffic from instances that actually possess an IPv6 address; without an IPv6 address on the instance’s elastic network interface, the gateway has no source to route from, even with a correct ::/0 route in the private subnet’s route table. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that an egress-only internet gateway is purely an IPv6 construct and does not perform any address translation—it relies entirely on the instance having a globally unique IPv6 address. A common trap is assuming the route table alone is sufficient, but the missing piece is the instance-level IPv6 assignment, often overlooked when auto-assign IPv6 settings are disabled. Memory tip: No address, no egress—IPv6 outbound requires an IPv6 source.
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 IPv4 and IPv6 CIDRs. They have a public subnet with an internet gateway and a private subnet with a NAT gateway. EC2 instances in the private subnet need to download updates from the internet. The instances have IPv6 addresses. The private subnet route table has a default route (::/0) pointing to an egress-only internet gateway. However, instances cannot reach IPv6 internet destinations. The egress-only internet gateway is attached to the VPC and in 'available' state. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The instances do not have an IPv6 address assigned
Egress-only internet gateway is for outbound-only IPv6 traffic. It requires a route in the subnet route table. The route is present. However, egress-only internet gateway does not support IPv6 traffic that originates from the private subnet if the instances have public IPv6 addresses? Actually, egress-only internet gateway works for instances with IPv6 addresses. The issue might be that the instances have IPv6 addresses but the route table for the private subnet does not have a route to the egress-only internet gateway. But the question says it does. Another common issue: the egress-only internet gateway must be the target of the default IPv6 route. The most likely cause is that the route table for the public subnet (not private) does not have a route to the internet gateway for IPv6, but that's for public subnet. For private subnet, egress-only internet gateway is correct. But instances may need to have IPv6 addresses assigned. Option B (NAT gateway) is for IPv4 only. Option C (security group) is not likely. Option D (auto-assign IPv6) might be disabled.
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 security group for the instances is blocking outbound IPv6 traffic
Why it's wrong here
Security groups allow outbound by default.
✗
The NAT gateway does not support IPv6
Why it's wrong here
NATgateway is IPv4 only, but they are using egress-only internet gateway for IPv6.
✓
The instances do not have an IPv6 address assigned
Why this is correct
Without IPv6 address, instances cannot use egress-only internet gateway.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The egress-only internet gateway is not attached to the VPC
Why it's wrong here
It is attached and available.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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: The instances do not have an IPv6 address assigned — Egress-only internet gateway is for outbound-only IPv6 traffic. It requires a route in the subnet route table. The route is present. However, egress-only internet gateway does not support IPv6 traffic that originates from the private subnet if the instances have public IPv6 addresses? Actually, egress-only internet gateway works for instances with IPv6 addresses. The issue might be that the instances have IPv6 addresses but the route table for the private subnet does not have a route to the egress-only internet gateway. But the question says it does. Another common issue: the egress-only internet gateway must be the target of the default IPv6 route. The most likely cause is that the route table for the public subnet (not private) does not have a route to the internet gateway for IPv6, but that's for public subnet. For private subnet, egress-only internet gateway is correct. But instances may need to have IPv6 addresses assigned. Option B (NAT gateway) is for IPv4 only. Option C (security group) is not likely. Option D (auto-assign IPv6) might be disabled.
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.
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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