A subnet in my virtual private cloud (VPC) has run out of available IP addresses, and I'm using that subnet in conjunction with Elastic Load Balancing load balancers.

If subnets in your VPC run out of available IP addresses, AWS resources, such as load balancers, might not respond successfully to increased traffic.

It's a best practice to keep at least eight IP addresses in each subnet available for use. There are two ways to free up or add additional IP addresses for use with load balancers. The following considerations apply to both Application Load Balancers and Classic Load Balancers:

  • Delete unused elastic network interfaces (ENIs) to free up IP addresses in the subnet.
  • Create and add a new subnet to your VPC.

Note: Keep the requirements for subnets on a load balancer in mind—specifically, that load balancers can have at most one subnet per Availability Zone.  

To delete unused ENIs

To delete an unused ENI, see Deleting an Elastic Network Interface.

To add a new subnet with IP addresses available to your load balancer

  1. Create and add a new subnet to your VPC. Note: You can create a new subnet using the VPC's original CIDR blocks, or add additional CIDR blocks to your VPC for use with the new subnet.
  2. Replace your old subnet with the new subnet. For Classic Load Balancers, see Add a Subnet. For Application Load Balancers, see Availability Zones for Your Application Load Balancer.
  3. Review the route tables and access control list (ACL) rules associated with your subnet to be sure that your new subnet routes traffic the same way your previous subnet did. For example, if your previous subnet had a default route configured to an internet gateway, be sure that your new subnet has a similar default route.
  4. (Optional) Enable cross-zone load balancing as a best practice.

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Published: 2016-11-02

Updated: 2018-05-21