AWS Certified Solutions Architect – Professional — Question 595

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 20.0.1.0/24.
How can the user create the second subnet?

Answer options

• A. The user can modify the first subnet CIDR with AWS CLI
• B. The user can modify the first subnet CIDR from the console
• C. 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
• D. It is not possible to create a second subnet with overlapping IP CIDR without deleting the first subnet.

Correct answer: D

Explanation

In AWS VPC, once a subnet is created, its CIDR block cannot be modified or resized. Because the first subnet occupies the entire 20.0.0.0/16 range, any attempt to create a second subnet within that range will fail due to overlapping IP addresses. The only solution is to delete the original subnet and then recreate the subnets with non-overlapping, smaller CIDR blocks.