ANS-C01 Network Design Practice Question
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?
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 VPC endpoint's route table does not include the subnet's route table, so traffic to S3 is not routed through the endpoint.
The most likely cause is that the VPC endpoint's route table does not include the subnet's route table, meaning traffic from the private subnet to S3 is not routed through the endpoint. Option B is correct because if the subnet's route table lacks a route for the S3 prefix list pointing to the VPC endpoint, traffic will go via a NAT gateway or internet gateway (if configured) or fail if no such connectivity exists. Option A is incorrect because network ACLs are not mentioned and typically allow outbound HTTPS. Option C is incorrect since the security group allows all outbound traffic. Option D is incorrect because the bucket policy already permits access from the VPC endpoint.
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.
- ✗
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.
Visual reference
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
Go deeper
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.