SOA-C02 Cost and Performance Optimization Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  Use Elastic IP addresses for all instances

  Why it's wrong here

  Assigning Elastic IP addresses to instances does not reduce data transfer costs; it can increase them. Elastic IPs are public IPv4 addresses, and when instance-to-instance traffic is sent over these public IPs, it routes through the internet gateway and is billed at public internet data transfer rates (typically $0.09–$0.12 per GB). Additionally, Elastic IPs themselves incur an hourly cost when not attached to a running instance, and they are not designed for intra-VPC communication — you should instead use private IPs, which have no such charges.

• ✗

  Place instances in public subnets and route traffic through a NAT Gateway

  Why it's wrong here

  Placing instances in public subnets and routing traffic through a NAT Gateway would actively increase data transfer costs. A NAT Gateway is intended only for outbound internet access from private subnets; using it for instance-to-instance traffic would force packets to leave the VPC, incur both NAT Gateway hourly processing fees and per-GB data processing charges, and add unnecessary latency. Intra-VPC traffic between instances already traverses the internal network without needing any gateway, so routing it through a NAT Gateway is both expensive and functionally incorrect.

• ✓

  Ensure instances communicate using private IP addresses within the same VPC

  Why this is correct

  Ensuring instances communicate using private IP addresses within the same VPC is the correct and most cost-effective approach. Traffic sent over private IPv4 addresses stays entirely inside the VPC, never crossing the internet gateway, so it is not subject to public internet data transfer rates. For instances in the same Availability Zone, this traffic is absolutely free; even across Availability Zones, the per-GB charge is only $0.01 each way, which is far cheaper than public IP or gateway-based alternatives.

• ✗

  Use VPC endpoints to communicate between instances

  Why it's wrong here

  VPC endpoints are not a valid mechanism for reducing instance-to-instance data transfer costs. Gateway endpoints (S3, DynamoDB) and interface endpoints (other AWS services) are used exclusively to privately connect your VPC to supported AWS services, not to route traffic between EC2 instances. Using a VPC endpoint for instance communication would be architecturally unsupported, and interface endpoints additionally incur hourly charges and per-GB data processing fees, making them entirely counterproductive to the stated cost-reduction goal.

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