Automating CIDR expansion: Reducing IP exhaustion downtime

This article presents a serverless automation solution that monitors VPC IP utilization and automatically expands CIDR blocks within minutes to prevent IP exhaustion downtime.

• Combines Lambda, Step Functions, CloudWatch, IPAM, and DynamoDB to automate VPC CIDR expansion triggered by utilization thresholds
• Supports three operating modes: full_ipam for IPAM-managed address space, ipam_subnets_only for external VPC CIDR management, and no_ipam for environments without IPAM
• Automatically discovers existing subnet naming conventions, route tables, and network ACLs to apply to newly created subnets
• Uses DynamoDB distributed locking to prevent concurrent expansions and records all expansions for auditing
• Deployed via AWS SAM with VPC tag-based configuration; no per-VPC alarm setup required
• Includes retry logic, error handling, and SNS notifications for expansion success or failure

The solution reduces operational overhead by automating manual CIDR expansion processes that typically take hours, enabling applications to maintain capacity during traffic spikes.