What is the minimum size subnet you can have in an Amazon VPC?

The minimum size subnet you can have in an Amazon VPC is a /28 subnet. A subnet's CIDR notation indicates the number of bits used for the network portion of the address, with the remainder used for host addresses.

In a /28 subnet, there are 32 total IP addresses, of which 14 are usable for hosts because 2 addresses are reserved (one for the network itself and one for the broadcast address). This means that while /28 is the smallest CIDR block you can create in a VPC, it still provides enough usable addresses for a small number of instances and services.

Larger subnets, such as /24 or /20, provide more IP addresses but the /28 subnet is the smallest valid size that allows the creation of an operational subnet in Amazon VPC. Meanwhile, a /30 subnet, although small, only allows for 2 usable IP addresses, which limits it primarily to point-to-point connections and is typically not used for general subnet purposes in VPC contexts. Thus, while smaller CIDR notations might exist theoretically, /28 is the correct and accepted minimum size for creating a subnet within Amazon VPC.