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What is CIDR?

Networks need a way to group IP addresses together, and they do that with CIDR.

CIDR stands for Classless Inter-Domain Routing. It’s a method for defining a range of IP addresses using a single, compact notation. You’ve probably seen it written as something like “192.168.1.0/24,” where the number after the slash tells you how many addresses are included in that range.

Before CIDR was developed in 1993, IP addresses were grouped into rigid, predetermined classes that often wasted large blocks of addresses on networks that didn’t even need them. CIDR replaced that system with something more flexible. This allows network administrators to define ranges that fit their actual needs rather than defaulting to a fixed size.

In practice, CIDR shows up in a few places. Firewalls use CIDR notation to block or allow traffic from an entire range of addresses at once rather than listing each one individually. BGP uses it to advertise which IP ranges a network is responsible for. And if you’ve ever looked up an IP address and seen a network range listed alongside it, that range is expressed in CIDR notation.

For most users, CIDR stays in the background, yet it’s a fundamental part of how the internet organizes and routes traffic at scale.