The answer is that the route to the Gateway VPC Endpoint was added only to the public subnets, not to the private subnets where the EC2 instances reside. For a Gateway VPC Endpoint to route traffic from private subnets directly to S3, the route table associated with each private subnet must contain a specific route pointing the S3 prefix list (e.g., com.amazonaws.region.s3) to the endpoint ID. Without this route, traffic from the EC2 instances follows the default route (0.0.0.0/0) through a NAT Gateway or Internet Gateway, bypassing the endpoint and incurring data transfer costs. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that Gateway Endpoints are regional and require explicit subnet-level routing—a common trap is assuming that adding the route to public subnets alone suffices. Remember the memory tip: “Private instances need their own endpoint route; public routes don’t help private traffic.”
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 a production VPC with CIDR 10.0.0.0/16. They have an internet-facing Application Load Balancer (ALB) in public subnets across two Availability Zones. The ALB distributes traffic to a fleet of EC2 instances in private subnets. The EC2 instances need to access an Amazon S3 bucket to retrieve configuration files. The company wants to minimize data transfer costs and ensure that traffic to S3 does not traverse the internet. A network engineer created a Gateway VPC Endpoint for S3 in the VPC and added a route in the public subnet route tables pointing to the endpoint. However, the EC2 instances still cannot access the S3 bucket. The security groups for the EC2 instances allow outbound HTTPS to 0.0.0.0/0. The NACLs are default (allow all). The S3 bucket policy allows access from the VPC endpoint. What is the MOST likely reason the EC2 instances cannot access S3?
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.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route to the Gateway VPC Endpoint was added only to public subnets, not to private subnets
Option A is correct. The Gateway VPC Endpoint is only accessible from within the VPC, but the route to the endpoint must be added to the route tables of the subnets where the EC2 instances reside. The engineer added the route only to public subnet route tables, not to the private subnet route tables. Therefore, traffic from the EC2 instances destined to S3 uses the default route (0.0.0.0/0) which goes through the NAT Gateway or Internet Gateway, not through the endpoint. Option B is incorrect because the security group rule allows outbound HTTPS to all destinations. Option C is incorrect because the S3 bucket policy allows the VPC endpoint. Option D is incorrect because the endpoint is in the same region as the bucket.
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 Gateway VPC Endpoint is in a different region than the S3 bucket
Why it's wrong here
Incorrect; Gateway Endpoints are regional and S3 bucket is in the same region.
✗
The security group for the EC2 instances does not allow outbound traffic to the S3 prefix list
Why it's wrong here
Incorrect; the security group allows outbound HTTPS to 0.0.0.0/0, which includes S3.
✗
The S3 bucket policy does not include a condition for the VPC endpoint
Why it's wrong here
Incorrect; the bucket policy allows access from the VPC endpoint.
✓
The route to the Gateway VPC Endpoint was added only to public subnets, not to private subnets
Why this is correct
Correct; the route must be in the private subnet route table for EC2 instances to use the endpoint.
Clue confirmation
The clue words "most likely", "minimum / minimize" in the question point toward this answer.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
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 route to the Gateway VPC Endpoint was added only to public subnets, not to private subnets — Option A is correct. The Gateway VPC Endpoint is only accessible from within the VPC, but the route to the endpoint must be added to the route tables of the subnets where the EC2 instances reside. The engineer added the route only to public subnet route tables, not to the private subnet route tables. Therefore, traffic from the EC2 instances destined to S3 uses the default route (0.0.0.0/0) which goes through the NAT Gateway or Internet Gateway, not through the endpoint. Option B is incorrect because the security group rule allows outbound HTTPS to all destinations. Option C is incorrect because the S3 bucket policy allows the VPC endpoint. Option D is incorrect because the endpoint is in the same region as the bucket.
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", "minimum / minimize". 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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