In networking, 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.
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.
Therefore, the prefix length of a CIDR block determines the size of a network by specifying the number of IP addresses that can be used within the network.