AWS Certified SysOps Administrator – Associate (legacy) — Question 344

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 by mistake. The user is trying to create another subnet of CIDR 20.0.0.1/24. How can the user create the second subnet?

Answer options

• A. There is no need to update the subnet as VPC automatically adjusts the CIDR of the first subnet based on the second subnet's CIDR
• B. The user can modify the first subnet CIDR from the console
• C. It is not possible to create a second subnet as one subnet with the same CIDR as the VPC has been created
• D. The user can modify the first subnet CIDR with AWS CLI

Correct answer: C

Explanation

Once a subnet is created in an AWS VPC, its CIDR block cannot be modified or resized. Since the first subnet consumes the entire VPC CIDR block (20.0.0.0/16), there are no remaining IP addresses available to allocate to a second subnet. To resolve this, the user must delete the existing subnet and recreate it with a smaller CIDR block before provisioning additional subnets.