Creates a default subnet with a size <code>/20</code> IPv4 CIDR block in the specified Availability Zone in your default VPC. You can have only one default subnet per Availability Zone. For more information, see <a href="https://docs.aws.amazon.com/vpc/latest/userguide/default-vpc.html#create-default-subnet">Creating a default subnet</a> in the <i>Amazon Virtual Private Cloud User Guide</i>.