How can the user create the second subnet?
The user can modify the first subnet CIDR with AWS CLI
The user can modify the first subnet CIDR from the console
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
It is not possible to create a second subnet with overlapping IP CIDR without deleting the first subnet.
Explanations:
AWS does not allow modification of a subnet’s CIDR block once it is created. The subnet CIDR must remain fixed and cannot be changed directly through the AWS CLI or the console.
Similar to option A, AWS does not permit changes to the CIDR block of an existing subnet. Once created, the subnet’s CIDR is immutable.
AWS does not automatically adjust the CIDR of an existing subnet to accommodate a new subnet. Subnets within a VPC must have non-overlapping CIDR ranges.
It is indeed not possible to create a second subnet with an overlapping CIDR range (20.0.1.0/24) without first deleting or modifying the existing subnet (20.0.0.0/16) because subnets within the same VPC must have unique and non-overlapping CIDR blocks.
I outline that the answer is:
It is not possible to create a second subnet with overlapping IP CIDR without deleting the first subnet.