In the sizing example, what is the minimum CIDR block for each VPC?

• A. /23
• B. /22
• C. /24
• D. /16

Answer: (A)

The idea being tested is sizing a VPC so it can hold all required subnets across Availability Zones. If you plan to have two subnets—one in each of two AZs—each sized as a /24, your VPC must cover both of those blocks. A /23 gives you 512 IP addresses, which is enough to contain those two /24 subnets within the same VPC space. After AWS reserves addresses in each subnet, you still have ample usable addresses overall, and you have room to grow without needing a larger VPC than necessary. A /24 would fit only one /24 subnet, so it wouldn’t accommodate two subnets. A /22 or larger would work, but they provide more space than needed; the question asks for the smallest viable size.