Amazon VPC gives you your own private space in the cloud. When you create a VPC, you have the option of carving out your own data center in the cloud.
The first step of creating a VPC is deciding the IP range by providing a Classless Inter-Domain Routing (CIDR) block.
VPC now supports both IPv4 and IPv6, so you can have both IP ranges as part of your VPC
. When you choose an IPv4 CIDR range, you can choose anything between /16, which corresponds to 65,536 IP addresses (for example 10.0.0.0/16), and /28, which corresponds to 16 IP addresses.
If you choose IPv6, the size of the IPv6 CIDR block is fixed to /56. The range of IPv6 addresses is automatically allocated from Amazon's pool of IPv6 addresses; at this time, you cannot select the range yourself. As of now, having a CIDR block for IPv6 is optional; however, you need an IPv4 CIDR block. It is important to note that once you create a VPC, you can't alter the size of it. If you create a VPC with a small size and later realize that you need more IP addresses, you can create a new VPC with a bigger IP address range and then migrate your applications from the old VPC to the new one.
A VPC is limited to a region, which means you can't have a VPC spanning regions. Within a VPC, you have all the AZs that are part of the region where the VPC belongs. VPC spanning three AZs within a region with a CIDR block of /16. This figure also shows the main route table of the virtual private cloud
source: google
