AWS Certified SysOps Administrator – Associate (legacy) — Question 330

A user has created a VPC with CIDR 20.0.0.0/16. The user has created one subnet with CIDR 20.0.0.0/16 in this VPC. The user is trying to create another subnet with the same VPC for CIDR 20.0.0.1/24. What will happen in this scenario?

Answer options

• A. The VPC will modify the first subnet CIDR automatically to allow the second subnet IP range
• B. It is not possible to create a subnet with the same CIDR as VPC
• C. The second subnet will be created
• D. It will throw a CIDR overlaps error

Correct answer: D

Explanation

In AWS VPC, subnet CIDR blocks must not overlap with any existing subnets within the same VPC. Since the first subnet was allocated the entire 20.0.0.0/16 range, attempting to create a second subnet with 20.0.0.1/24 will fail and trigger a CIDR overlap error. AWS does not automatically modify existing subnet CIDRs to prevent conflicts.