- A
The security group for the instances is blocking outbound IPv6 traffic
Correct. The security group may have outbound rules that block IPv6 traffic, preventing instances from reaching IPv6 internet destinations despite proper gateway and route configuration.
- B
The NAT gateway does not support IPv6
Why wrong: Incorrect. NAT gateway does not support IPv6, but the route is pointing to an egress-only internet gateway, not a NAT gateway. This is not the cause.
- C
The instances do not have an IPv6 address assigned
Why wrong: Incorrect. The stem explicitly states that the instances have IPv6 addresses assigned.
- D
The egress-only internet gateway is not attached to the VPC
Why wrong: Incorrect. The stem states that the egress-only internet gateway is attached to the VPC and in 'available' state.
ANS-C01 Egress-Only Internet Gateway 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. A key principle to apply: egress-Only Internet Gateway. 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 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 security group for the instances is blocking outbound IPv6 traffic
The egress-only internet gateway is correctly configured (attached to VPC and route rule ::/0 exists). The instances have IPv6 addresses as stated. The most likely remaining cause is that the security group attached to the instances has outbound rules blocking IPv6 traffic. By default, security groups allow all outbound traffic, but if a custom rule was applied, it could restrict IPv6 traffic. Option B is irrelevant because NAT gateway is for IPv4. Option C contradicts the stem. Option D contradicts the stem.
Key principle: Egress-Only Internet Gateway
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 this is correct
Correct. The security group may have outbound rules that block IPv6 traffic, preventing instances from reaching IPv6 internet destinations despite proper gateway and route configuration.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Egress-Only Internet Gateway
- ✗
The NAT gateway does not support IPv6
- ✗
The instances do not have an IPv6 address assigned
Why it's wrong here
Incorrect. The stem explicitly states that the instances have IPv6 addresses assigned.
- ✗
The egress-only internet gateway is not attached to the VPC
Why it's wrong here
Incorrect. The stem states that the egress-only internet gateway is attached to the VPC and in 'available' state.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Candidates often overlook security group rules when troubleshooting outbound connectivity, especially for IPv6. Ensure security group outbound rules allow the desired traffic.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- Egress-Only Internet Gateway
- Security Group
- Route Table
- NAT Gateway
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Egress-Only Internet Gateway
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review egress-Only Internet Gateway, then practise related ANS-C01 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Implementation — This question tests Network Implementation — Egress-Only Internet Gateway.
What is the correct answer to this question?
The correct answer is: The security group for the instances is blocking outbound IPv6 traffic — The egress-only internet gateway is correctly configured (attached to VPC and route rule ::/0 exists). The instances have IPv6 addresses as stated. The most likely remaining cause is that the security group attached to the instances has outbound rules blocking IPv6 traffic. By default, security groups allow all outbound traffic, but if a custom rule was applied, it could restrict IPv6 traffic. Option B is irrelevant because NAT gateway is for IPv4. Option C contradicts the stem. Option D contradicts the stem.
What should I do if I get this ANS-C01 question wrong?
Review egress-Only Internet Gateway, then practise related ANS-C01 questions on the same topic to reinforce the concept.
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?
Egress-Only Internet Gateway
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Last reviewed: Jun 20, 2026
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