Understanding AWS Lambda

AWS Lambda is a serverless compute service that allows you to run code without provisioning or managing servers. It automatically scales your applications by running code in response to triggers such as changes in data, shifts in system state, or user actions.

Identifying the Symptom: SubnetIPAddressLimitReachedException

When deploying or running a Lambda function, you might encounter the error SubnetIPAddressLimitReachedException. This error indicates that the subnet associated with your Lambda function has reached its maximum IP address capacity.

What You Observe

Typically, this error manifests when you attempt to execute a Lambda function that is configured to run within a VPC (Virtual Private Cloud), and the subnet does not have enough available IP addresses to assign to the function's ENI (Elastic Network Interface).

Exploring the Issue: Why It Happens

The SubnetIPAddressLimitReachedException occurs because each subnet in a VPC has a limited number of IP addresses. When this limit is reached, AWS cannot allocate additional IP addresses for new ENIs, which are required for Lambda functions to communicate within the VPC.

Technical Explanation

Each subnet is defined by a CIDR block, which determines the range of IP addresses available. If your subnet's CIDR block is too small, or if there are too many resources consuming IP addresses, you will encounter this limitation.

Steps to Resolve the SubnetIPAddressLimitReachedException

To resolve this issue, you can either increase the size of your subnet or use a different subnet with available IP addresses. Here are the steps to achieve this:

Option 1: Increase Subnet Size

1. Identify the subnet causing the issue by checking the VPC configuration in the AWS VPC Console.
2. Check the current CIDR block of the subnet. Consider expanding it if possible. Note that modifying a subnet's CIDR block is not directly possible; you may need to create a new, larger subnet.
3. Create a new subnet with a larger CIDR block within the same VPC.
4. Update your Lambda function's configuration to use the new subnet.

Option 2: Use a Different Subnet

1. Identify other subnets within your VPC that have available IP addresses.
2. Update your Lambda function's configuration to use one of these subnets.
3. Ensure that the new subnet has the necessary route tables and security group configurations to allow your Lambda function to operate correctly.

Additional Resources

For more information on managing VPCs and subnets, refer to the Amazon VPC Documentation. For details on configuring Lambda functions within a VPC, see the AWS Lambda VPC Configuration Guide.