CIDR

An IP prefix notation where the number after the slash indicates the length of the shared network portion in bits.

In plain words

CIDR describes a range of IP addresses: for example, 192.168.1.0/24 is 256 addresses from .0 to .255.

Practical example

The notation 10.0.0.0/8 means the entire network from 10.0.0.0 to 10.255.255.255 — over 16 million addresses.

Where you encounter it

When configuring firewalls, routing rules, and reading GeoIP data.