Previously used Address Assignment strategy, before adopting CIDR.
It constrained Subnets to have the following possibilities of IP Address length:
Class A: 8 bitsClass B:16 bitsClass C:24 bits
Problems
It turned out to be problematic since class A subnets could have 254 hosts, which is too small for many organisations, while class B could have 65.634 hosts which is too large. This mean that many addresses in class B subnets were unused and therefore resulted in poor utilisation of the address space.