The answer is a missing route table association in the S3 Gateway Endpoint. When you create a gateway endpoint for S3, you must explicitly associate it with the route tables used by your subnets; without this association, the endpoint’s prefix list route for S3 is never added to those route tables, so traffic from your EC2 instances in the private subnet still goes out to the internet or NAT gateway instead of through the VPC endpoint. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that a VPC endpoint is not automatically active—it requires a deliberate route table association to direct S3 traffic internally. A common trap is assuming that simply creating the endpoint or allowing outbound security group rules is enough, but the routing layer is the critical missing link. Remember the memory tip: “No route, no route”—if the endpoint’s route table doesn’t match the subnet’s route table, S3 traffic takes the wrong path.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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.
An application running on EC2 instances in a VPC needs to access an Amazon S3 bucket to read configuration files. The VPC has an S3 VPC endpoint configured. The instances are in a private subnet and have a security group that allows all outbound traffic. The bucket policy allows access from the VPC endpoint. However, the application fails to 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 VPC endpoint's route table does not include the subnet's route table, so traffic to S3 is not routed through the endpoint.
Option A is correct. The VPC endpoint must be associated with a route table that includes the subnet's route table. If not, traffic from the subnet to S3 will not use the endpoint. Option B is incorrect because security group outbound is all traffic. Option C is incorrect because the bucket policy allows access from the endpoint. Option D is incorrect because there is no NACL mentioned, and if NACL blocks, it would block all traffic.
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 network ACL for the private subnet is blocking outbound HTTPS traffic.
Why it's wrong here
No mention of NACL; if it blocked, it would be a likely cause but not most likely given other clues.
✓
The VPC endpoint's route table does not include the subnet's route table, so traffic to S3 is not routed through the endpoint.
Why this is correct
VPC endpoints require route table association to route traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group for the EC2 instances has an outbound rule that blocks HTTPS traffic to S3.
Why it's wrong here
The scenario says all outbound traffic is allowed.
✗
The S3 bucket policy does not grant access to the VPC endpoint's ID.
Why it's wrong here
The bucket policy allows access from the endpoint.
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.
Trap categories for this question
Scenario analysis trap
The scenario says all outbound traffic is allowed.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The VPC endpoint's route table does not include the subnet's route table, so traffic to S3 is not routed through the endpoint. — Option A is correct. The VPC endpoint must be associated with a route table that includes the subnet's route table. If not, traffic from the subnet to S3 will not use the endpoint. Option B is incorrect because security group outbound is all traffic. Option C is incorrect because the bucket policy allows access from the endpoint. Option D is incorrect because there is no NACL mentioned, and if NACL blocks, it would block all traffic.
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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