CIDR & Subnetting FAQ What is CIDR? Classless Inter-Domain Routing (CIDR) is a compact way to write IP address ranges and their network masks. Why is CIDR notation useful? It lets network engineers allocate address space precisely, aggregate routes and keep routing tables smaller. How is an IP address structured? An IPv4 address consists of four 8-bit numbers (octets) ranging from 0 to 255, separa