CIDR notation

CIDR notation is a compact representation of an IP address and its associated routing prefix. The notation is constructed from an IP address, a slash ('/') character, and an integer. The integer is the count of leading 1 bits in the subnet mask. Larger values here indicate smaller networks.

Classful network design for IPv4 sized the network prefix as one or more 8-bit groups, resulting in the blocks of Class A, B, or C addresses. IP address allocations were based on the bit boundaries of the four octets of an IP address. (The other two classes are used for other purposes – class D for multicast and class E for experimental purposes.)

Classless addressing is an IP address allocation method that is designed to replace classful addressing to minimize the rapid exhaustion of IP addresses.

Class A addresses allocate first 8 bits for the network and the remaining bits for the host.
Class B addresses allocate first 16 bits for the network and the remaining bits for the host.
Class C addresses allocate first 24 bits for the network and the remaining bits for the host.

What used to be class A is now '/8', B is '/16', C is '/24' and '/32' is the 'netmask' for a single host.
CIDR sees an IP address as a 32 bit rather than a 4 bytes address.

https://www.ietf.org/rfc/rfc1517.txt
Classless Inter-Domain Routing (CIDR)

http://jodies.de/ipcalc
IP Calculator