Final answer:
The IP CIDR of a default VPC in AWS is preset and often is 172.31.0.0/16, but it may vary by region. AWS assigns a large CIDR block to the default VPC to accommodate a substantial number of IP addresses. Custom VPCs, however, allow users to configure their own CIDR range.
Step-by-step explanation:
The IP CIDR notations of the default VPC depend on the AWS region you are operating in. Each new AWS account is provided with a default VPC in every region, which includes a default subnet in each Availability Zone. The default VPC is usually configured with a large CIDR block, often 172.31.0.0/16, providing up to 65,536 IP addresses. However, this CIDR block is not universal and can vary by region.
The creation of a custom VPC allows the user to specify their own CIDR range that can enable more precise network configurations fitting their business needs. While the default CIDR of a default VPC is preset by AWS and generally the same across most regions, custom VPCs can be configured with a CIDR range as per the user's planning and requirements within the IPv4 private address space limitations.