Creates a subnet in an existing VPC.
When you create each subnet, you provide the VPC ID and the CIDR block you want for the subnet. After 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 /16 netmask (65,536 IP addresses).
AWS reserves both the first four and the last IP address in each subnet's
CIDR block. They're not available for use.
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 launch an instance in a VPC using an Amazon EBS-backed AMI, the IP address doesn't change if you stop and restart the instance (unlike a similar instance launched outside a VPC, which gets a new IP address when restarted). It's therefore possible to have a subnet with no running instances (they're all stopped), but no remaining IP addresses available.
For more information about subnets, see Your VPC and Subnets (http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html) in the Amazon Virtual Private Cloud User Guide.