VPC subnet calculator
Enter an IPv4 CIDR block. Everything is calculated in your browser — nothing is sent anywhere. The AWS row is the one most calculators leave out: AWS reserves five addresses in every subnet, so a /24 gives you 251 usable addresses, not 254.
Result
Split this block
The first four subnets you get by splitting into the selected size.
| Subnet | First | Last | AWS usable |
|---|
How the AWS number is worked out
AWS reserves five addresses in every VPC subnet: the network address, the VPC router (base + 1), the Amazon-provided DNS resolver (base + 2), one address held for future use (base + 3), and the broadcast address at the end. So the usable count is 2^(32 − prefix) − 5, and AWS only accepts subnet prefixes from /16 to /28.
Remember that a NAT gateway, each interface VPC endpoint, every load balancer node, and — under the AWS VPC CNI — every individual Kubernetes pod consumes one of those addresses. Subnet prefixes cannot be changed after creation, so size upward.