Before CIDR, Internet routers used an inefficient IP addressing scheme based on classes. Organizations like ISPs reserved address blocks in large "Class A," "Class B," or "Class C" chunks that wasted much of the IP address range.
In contrast, CIDR makes the IP addressing space classless. CIDR associates network masks with IP network numbers independent of their traditional class. Routers that support CIDR recognize these networks as individual routes, even though they may represent an aggregation of several traditional subnets.