Classful addressing is an early method of organizing IP Addresses in the IPv4 system, dividing the address space into five classes (A, B, C, D, and E) based on the first few bits of the address. This system was used from 1981 until it was largely replaced by Classless Inter-Domain Routing (CIDR) in 1993 due to inefficiencies and limitations in address allocation. Each class has a fixed network size, i.e. fixed Subnet Mask size. In this system, there was a lot of waste. If you needed a Network with 300 hosts, you could not be given a Class C (/24 → 256 host) address range. Instead you would have to be given a Class B (/16 → 65536 host) address range.