IP Range to CIDR

It takes an arbitrary start and end IP address and returns the smallest set of CIDR blocks (network/prefix pairs like 192.168.1.0/24) that covers exactly that range — no more and no fewer addresses. This is the reverse of a CIDR calculator, which expands a single CIDR block into its address range. It is commonly needed for firewall rules, route tables, ACLs, and allow/deny lists that accept CIDR notation but not raw ranges.

A single CIDR block can only describe a range whose size is a power of two and whose start is aligned to that size. An arbitrary range — for example 192.168.1.5 to 192.168.1.20 — usually is not power-of-two sized or aligned, so it must be split into several aligned power-of-two blocks. The converter always returns the minimal set: the fewest CIDR blocks that cover the range exactly.

Yes. The tool detects whether both addresses are IPv4 or IPv6 and runs the same aggregation algorithm using big-integer math, so an IPv6 range such as 2001:db8:: to 2001:db8::ff is reduced to its minimal CIDR set just like an IPv4 range. Both addresses must be the same family.