The answer is that the security group outbound rule does not allow traffic to the S3 prefix list. This is correct because VPC Gateway Endpoints operate using prefix lists—a logical grouping of IP addresses used by AWS services—and security group outbound rules must reference the specific prefix list ID for S3 (e.g., pl-xxxxx) rather than a CIDR range or 0.0.0.0/0. Even if the route table correctly points to the endpoint, the security group acts as a stateful virtual firewall that can block traffic if the destination is not explicitly permitted. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how gateway endpoints differ from interface endpoints, particularly the requirement to use prefix lists in security group rules instead of relying on NAT or internet gateways. A common trap is assuming that “all outbound traffic” in a security group automatically covers endpoint traffic, but the rule must specify the S3 prefix list. Memory tip: think “PL for S3” — Prefix List for S3 in the security group outbound rule.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 security engineer is troubleshooting connectivity between an EC2 instance in a private subnet and an S3 bucket. The instance has a VPC gateway endpoint for S3, and the route table has a route to the endpoint. The security group for the instance allows all outbound traffic. However, the instance cannot access the S3 bucket. 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 security group outbound rule does not allow traffic to the S3 prefix list.
Option A is correct because VPC gateway endpoints use prefix lists, and the security group outbound rule must allow traffic to the S3 prefix list ID. Option B is wrong because the instance is in a private subnet; internet traffic would go through NAT, but S3 access should use the gateway endpoint. Option C is wrong because S3 buckets are accessed via the endpoint, not via the internet. Option D is wrong because the route is present, but security group may still block.
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 outbound rule does not allow traffic to the S3 prefix list.
Why this is correct
Gateway endpoints are accessed via prefix list; security group must allow egress to that prefix list.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The VPC endpoint is not associated with the route table.
Why it's wrong here
The question states route table has a route to the endpoint, so association exists.
✗
The route table does not have a route to a NAT gateway.
Why it's wrong here
NATgateway is for internet access; S3 gateway endpoint works with private subnets.
✗
The S3 bucket policy does not allow access from the VPC endpoint.
Why it's wrong here
Bucket policy might also be an issue, but security group is more likely if outbound is unrestricted.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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 Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group outbound rule does not allow traffic to the S3 prefix list. — Option A is correct because VPC gateway endpoints use prefix lists, and the security group outbound rule must allow traffic to the S3 prefix list ID. Option B is wrong because the instance is in a private subnet; internet traffic would go through NAT, but S3 access should use the gateway endpoint. Option C is wrong because S3 buckets are accessed via the endpoint, not via the internet. Option D is wrong because the route is present, but security group may still block.
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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