CIDR Calculator

Convert in either direction. Give a CIDR block such as 192.168.1.0/24 to see the address range it covers, or give a range such as 192.168.1.10 - 192.168.1.200 to get the smallest set of CIDR blocks that covers it exactly.

The second direction is the awkward one to do by hand, and it is what you need when translating a range from a firewall ticket into rules a router will accept.

How it works

CIDR notation writes a block of addresses as a starting address and a prefix length: 192.168.1.0/24 means "the first 24 bits are fixed, the remaining 8 vary", which is 256 addresses. It replaced the old class-based system where block sizes were fixed by the leading bits of the address.

The constraint that makes range-to-CIDR interesting is that a CIDR block cannot start anywhere. Every block must begin on a boundary that is a multiple of its own size — a /24 must start on a multiple of 256, a /26 on a multiple of 64. So an arbitrary range like .10 to .200 cannot be one block; it must be decomposed into several aligned blocks of varying sizes.

The algorithm is greedy: at the current position, take the largest block that both starts on a valid boundary for its size and does not overshoot the end of the range; move to the next address after it; repeat. That produces the minimum number of blocks covering the range exactly, with no addresses outside it.

This matters in practice because getting it wrong is silent and dangerous. It is tempting to round a range up to a convenient block — but 192.168.1.10/24 does not mean "from .10 onward"; it means the whole 192.168.1.0/24, including the ten addresses you meant to exclude. In a firewall allowlist, that is a quietly wider hole than intended.

Frequently asked questions