Set up a log group in Amazon CloudWatch Logs for each subnet. Create an AWS Lambda function that reads each subnet's IP address usage and publishes metrics to the log group. Configure an Amazon CloudWatch alarm to send an Amazon Simple Notification Service (Amazon SNS) notification if the availability limit threshold is reached.

Set up a custom Amazon CloudWatch metric for IP address usage for each subnet. Create an AWS Lambda function that reads each subnet's IP address usage and publishes a CloudWatch metric dimension. Schedule the Lambda function to run every 5 minutes. Configure a CloudWatch alarm to send an Amazon Simple Notification Service (Amazon SNS) notification if the availability limit threshold is reached.

Set up Amazon VPC IP Address Manager (IPAM) with a new top-level pool. In the top-level pool, create a pool for each VPC. In each VPC pool, create a pool for each subnet in that VPC. Turn on the auto-import option for the VPC pools and the subnet pools. Configure an Amazon EventBridge rule that monitors each pool availability limit threshold and sends an Amazon Simple Notification Service (Amazon SNS) notification if the limit threshold is reached.