What Subnet CIDR Blocks Are Available?ΒΆ
A subnet CIDR block must be included in its VPC CIDR block. Supported VPC CIDR blocks are 10.0.0.0/8-24, 172.16.0.0/12-24, and 192.168.0.0/16-24. The allowed block size of a subnet is between the netmask of its VPC CIDR block and the /29 netmask.