How to Choose the Right AWS VPC CIDR Block Size

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When setting up a Virtual Private Cloud (VPC) in Amazon Web Services (AWS), it’s important to understand the restrictions around the size of the CIDR (Classless Inter-Domain Routing) block you choose. The CIDR block size must be between a /16 netmask, which provides 65,536 IP addresses, and a /28 netmask, which provides 16 IP addresses. Unfortunately, AWS does not allow creating a VPC with a /29 CIDR block or smaller.

Here are some key points to keep in mind:

– The CIDR block size must fall between /28 and /16.
– You cannot change the size of an existing CIDR block once it’s set.
– The CIDR block must not overlap with any other CIDR blocks already associated with the VPC.

If you need a network smaller than the minimum allowed /28, the best approach is to create a VPC with the smallest size (/28) and then divide it into subnets that meet your needs. Keep in mind that AWS reserves the first four IP addresses and the last IP address in each subnet’s CIDR range, so these addresses will not be available for your use.

This setup helps ensure your network configuration adheres to AWS policies while giving you flexibility within the provided limits.