The answer is that the EC2 instance is accessing S3 through a NAT Gateway instead of the VPC endpoint. This is because the S3 bucket policy includes a condition key like aws:SourceVpce that explicitly restricts access to requests originating from the specified VPC endpoint ID. When traffic routes through a NAT Gateway, the source VPC endpoint header is absent, so the condition fails and S3 denies the request. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how VPC endpoint conditions interact with routing paths—a common trap is assuming that any traffic from within the VPC is automatically allowed, but the policy only grants access when the request actually traverses the endpoint. Remember the memory tip: “If it doesn’t hit the endpoint, the condition won’t bend.”
ANS-C01 Network Implementation 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. 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 an S3 bucket with the bucket policy shown. The VPC endpoint ID is correct. However, an EC2 instance in a private subnet in the same VPC cannot download objects from the bucket. What is a possible reason?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The EC2 instance is accessing S3 via a NAT Gateway instead of the VPC endpoint
The bucket policy allows access only via the specified VPC endpoint. If the EC2 instance is accessing S3 through a NATGateway or internet gateway, the source VPC endpoint will not be present, and access will be denied. The instance should use the VPC endpoint.
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 EC2 instance does not have a route to the VPC endpoint
Why it's wrong here
VPC endpoints are accessible via the VPC's route table; if the endpoint is in the same VPC, no route is needed.
✗
The bucket policy does not allow s3:ListBucket
Why it's wrong here
The action is GetObject; ListBucket is not required for downloading.
✓
The EC2 instance is accessing S3 via a NAT Gateway instead of the VPC endpoint
Why this is correct
The policy only allows requests coming through the VPC endpoint; requests via NATGateway are denied.
The VPC endpoint security group is blocking traffic
Why it's wrong here
Security groups are not applicable to Gateway endpoints.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The EC2 instance is accessing S3 via a NAT Gateway instead of the VPC endpoint — The bucket policy allows access only via the specified VPC endpoint. If the EC2 instance is accessing S3 through a NAT Gateway or internet gateway, the source VPC endpoint will not be present, and access will be denied. The instance should use the VPC endpoint.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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