What is the largest CIDR network subnet block assignable for a VPC?

The largest CIDR (Classless Inter-Domain Routing) network subnet block assignable for a VPC (Virtual Private Cloud) is a /16. This means that a /16 subnet provides 65,536 IP addresses (from 0 to 65535) within that subnet, which allows for ample capacity for resources and instances within the VPC.

Using a /16 subnet also adheres to the AWS limits for VPCs, which typically provide a range of IP addresses that can adequately support a larger number of resources compared to smaller CIDR blocks. Smaller blocks such as /20, /18, or even /14 could be valid in certain contexts or configurations but would offer fewer usable IPs for a VPC, limiting scalability and resource allocation.

Therefore, choosing a /16 subnet block is optimal for networking within a VPC, ensuring maximum flexibility, resource management, and future growth potential. It allows you to create subnets without running out of IP addresses quickly, making it the preferred choice for larger deployments.