What Subnet CIDR Blocks Are Available?

The subnet CIDR blocks must be included in the VPC CIDR blocks. The VPC CIDR blocks are 10.0.0.0/8–24, 172.16.0.0/12–24, and 192.168.0.0/16–24. The subnet CIDR blocks must be within these CIDR blocks, and the allowed block size of a subnet is between the netmask of its VPC CIDR block and /29 netmask.