Associates a CIDR block with your VPC. You can associate a secondary IPv4 CIDR block, an Amazon-provided IPv6 CIDR block, or an IPv6 CIDR block from an IPv6 address pool that you provisioned through bring your own IP addresses (BYOIP). The IPv6 CIDR block size is fixed at /56.

You must specify one of the following in the request: an IPv4 CIDR block, an IPv6 pool, or an Amazon-provided IPv6 CIDR block.

For more information about associating CIDR blocks with your VPC and applicable restrictions, see VPC and subnet sizing in the Amazon Virtual Private Cloud User Guide.