The answer is the security group configuration, as the instance’s security group must include an inbound rule allowing IPv6 traffic from the internet. Even with an Internet Gateway (IGW) attached and a route for ::/0 pointing to the IGW in the public subnet’s route table, the security group acts as a virtual firewall that blocks all inbound traffic by default—including IPv6—unless explicitly permitted. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding that while an IGW supports both IPv4 and IPv6 inbound traffic, the security group’s default-deny behavior applies equally to both protocols. A common trap is assuming that adding an egress-only Internet Gateway (EIGW) is required for inbound IPv6, but EIGW only handles outbound traffic. To avoid this, remember the memory tip: “IGW for in, EIGW for out; security groups block all without a doubt.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. 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 both IPv4 and IPv6 CIDR blocks. They have a public subnet with an EC2 instance that needs to be accessible over IPv6 from the internet. The instance has an IPv6 address assigned. The SysOps administrator has attached an Internet Gateway (IGW) to the VPC and added a route to the IGW for the public subnet's IPv6 route table. However, the instance is not reachable over IPv6. What is the missing configuration?
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 attached to the instance does not allow inbound IPv6 traffic
Option B is correct because for IPv6 traffic, an egress-only Internet Gateway (EIGW) is required for outbound, but for inbound, an IGW supports both IPv4 and IPv6. Actually, IGW supports IPv6 inbound. The issue might be that the route table needs a route for ::/0 to the IGW. Option A is correct: The route table must have a route for all IPv6 traffic (::/0) to the IGW. Option B is wrong because EIGW is for outbound only. Option C is wrong because security groups allow inbound by default? No, they block inbound by default. So the security group must allow IPv6 traffic. Actually, both A and C could be correct. But the stem says they added a route to the IGW, but maybe they added a specific route? The most likely missing configuration is the security group rule. Let's check: The question says they added a route to the IGW for the public subnet's IPv6 route table. But the route must be for ::/0. If they added a specific route, that might be the issue. But the most common oversight is the security group. I'll go with Option C as the most likely. Option A is also plausible. To differentiate, the question says they added a route to the IGW, but doesn't specify if it's a default route. If they added a specific route, the instance might still be unreachable. However, the typical missing step is updating the security group. So I'll choose C.
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 instance does not have a public IPv6 address assigned
Why it's wrong here
The stem says it has an IPv6 address assigned.
✗
The route table for the subnet does not have a default route (::/0) to the IGW
Why it's wrong here
They added a route, but it might not be the default route; however, the more common issue is security.
✗
An egress-only Internet Gateway is required for IPv6
Why it's wrong here
EIGW is for outbound from private subnets; not needed for public subnet inbound.
✓
The security group attached to the instance does not allow inbound IPv6 traffic
Why this is correct
By default, security groups deny all inbound traffic; an explicit rule for IPv6 is needed.
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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Networking and Content Delivery — This question tests Networking and Content Delivery — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group attached to the instance does not allow inbound IPv6 traffic — Option B is correct because for IPv6 traffic, an egress-only Internet Gateway (EIGW) is required for outbound, but for inbound, an IGW supports both IPv4 and IPv6. Actually, IGW supports IPv6 inbound. The issue might be that the route table needs a route for ::/0 to the IGW. Option A is correct: The route table must have a route for all IPv6 traffic (::/0) to the IGW. Option B is wrong because EIGW is for outbound only. Option C is wrong because security groups allow inbound by default? No, they block inbound by default. So the security group must allow IPv6 traffic. Actually, both A and C could be correct. But the stem says they added a route to the IGW, but maybe they added a specific route? The most likely missing configuration is the security group rule. Let's check: The question says they added a route to the IGW for the public subnet's IPv6 route table. But the route must be for ::/0. If they added a specific route, that might be the issue. But the most common oversight is the security group. I'll go with Option C as the most likely. Option A is also plausible. To differentiate, the question says they added a route to the IGW, but doesn't specify if it's a default route. If they added a specific route, the instance might still be unreachable. However, the typical missing step is updating the security group. So I'll choose C.
What should I do if I get this SOA-C02 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 SOA-C02 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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