The answer is to implement a VPC Gateway Endpoint for S3, as this action most effectively reduces data transfer costs. This works because a gateway endpoint allows EC2 instances in private subnets to access S3 directly over the AWS internal network, eliminating the need for traffic to traverse a NAT gateway or internet gateway, which incurs per-GB data processing and transfer charges. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of cost optimization through network architecture, specifically the difference between gateway endpoints and interface endpoints, and the common trap is confusing CloudFront or NAT gateways as cost-saving solutions when they actually add or fail to eliminate transfer fees. A reliable memory tip is "Gateway for S3, free internally"—remember that S3 gateway endpoints are free to use and keep traffic within the AWS backbone, unlike NAT gateways which charge for both processing and data transfer.
SOA-C02 Cost and Performance Optimization Practice Question
This SOA-C02 practice question tests your understanding of cost and performance optimization. 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 SysOps administrator is troubleshooting a cost overrun in an AWS account. The cost explorer shows that data transfer costs have significantly increased. The architecture includes an Application Load Balancer (ALB) internet-facing, EC2 instances in private subnets, and an S3 bucket for static assets. Which action will MOST effectively reduce data transfer costs?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Implement a VPC Gateway Endpoint for S3 so that traffic from EC2 to S3 stays within the AWS network.
The correct answer is D. Data transfer from EC2 to S3 within the same region is free when using a VPC endpoint (gateway endpoint). Option A is wrong because CloudFront reduces latency but does not eliminate data transfer costs from ALB to EC2. Option B is wrong because moving to private ALB would require a VPN and may increase costs. Option C is wrong because NAT Gateway charges for data processing and transfer.
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.
✗
Enable Amazon CloudFront to cache static assets and reduce direct requests to the ALB.
Why it's wrong here
CloudFront reduces data transfer from origin but does not reduce ALB-to-EC2 costs.
✓
Implement a VPC Gateway Endpoint for S3 so that traffic from EC2 to S3 stays within the AWS network.
Why this is correct
Gateway endpoints are free and reduce data transfer costs to S3.
Replace the NAT Gateway with a NAT instance to reduce hourly charges.
Why it's wrong here
NATGateway data transfer costs are separate; this does not address ALB/EC2 costs.
✗
Change the ALB to internal (private) and use AWS Direct Connect for user access.
Why it's wrong here
This adds complexity and cost of Direct Connect.
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 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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Cost and Performance Optimization — This question tests Cost and Performance Optimization — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Implement a VPC Gateway Endpoint for S3 so that traffic from EC2 to S3 stays within the AWS network. — The correct answer is D. Data transfer from EC2 to S3 within the same region is free when using a VPC endpoint (gateway endpoint). Option A is wrong because CloudFront reduces latency but does not eliminate data transfer costs from ALB to EC2. Option B is wrong because moving to private ALB would require a VPN and may increase costs. Option C is wrong because NAT Gateway charges for data processing and transfer.
What should I do if I get this SOA-C02 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 SOA-C02 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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