Create Subnet Response from the service

You can create up to 20 subnets in a VPC. If you add more than one subnet to a VPC, they're set up in a star topology with a logical router in the middle. If you feel you need more than 20 subnets, you can request more by going to http://aws.amazon.com/contact-us/vpc-request/. When you create each subnet, you provide the VPC ID and the CIDR block you want for the subnet. Once you create a subnet, you can't change its CIDR block. The subnet's CIDR block can be the same as the VPC's CIDR block (assuming you want only a single subnet in the VPC), or a subset of the VPC's CIDR block. If you create more than one subnet in a VPC, the subnets' CIDR blocks must not overlap. The smallest subnet (and VPC) you can create uses a /28 netmask (16 IP addresses), and the largest uses a /18 netmask (16,384 IP addresses). *Important* AWS reserves both the first four and the last IP address in each subnet's CIDR block. They're not available for use. AWS might delete any subnet you create with this operation if you leave it inactive for an extended period of time (inactive means that there are no running Amazon EC2 instances in the subnet).