Each VPC can have one primary CIDR block, which cannot be modified after the VPC creation. When the IPs in the primary CIDR block can not meet your needs, you can create multiple secondary CIDR blocks to add IP ranges.
You can allocate the subnet with an IP range from the primary or secondary CIDR blocks. All subnets of the same VPC are interconnected by default, regardless of whether they belong to the primary or secondary CIDR blocks.
A secondary CIDR block can overlap with the destination IP range of a custom route. Note that the secondary CIDR block uses a local route, which has a higher priority than that of custom subnet routes.
Log in to the VPC console.
Select the region of the VPC at the top of the VPC page.
In the VPC list, locate the VPC from which secondary CIDR blocks will be deleted, and select More > Edit IPv4 CIDR block under the Operation column.
In the pop-up dialog box, click Delete next to the secondary CIDR block.