When you create a Virtual Private Cloud (VPC) in Amazon Web Services (AWS), you can specify a CIDR block for the VPC. A CIDR (Classless Inter-Domain Routing) block is a range of IP addresses that can be used to identify the devices within a network. It is written in the format X.X.X.X/YY, where X.X.X.X is the base IP address and YY is the prefix length, which determines the size of the network.
The prefix length specifies the number of bits in the IP address that are used to identify the network and the host. For example, a prefix length of 24 means that the first 24 bits of the IP address are used to identify the network, and the remaining 8 bits are used to identify the host. This results in a range of 256 addresses, which can be used to identify the devices within the network.
Specifying a CIDR block when creating a VPC allows you to specify the range of IP addresses that will be used within the VPC, and determines the size of the network. This can be useful for organizing and managing your resources within the VPC, and for defining security and access controls.