Fixed classes gave a company that needed 300 addresses a whole block of 65,534, and they made routing tables huge. CIDR lets a network be any prefix length, written like 10.0.0.0/22, and lets routers summarise many routes into one. Use CIDR notation whenever you define a range today, such as a cloud VPC or a firewall rule. Do not confuse it with subnetting; subnetting splits a block, while CIDR is the classless way of writing and combining blocks.