CIDR
Classless Inter-Domain Routing: a notation for IP address ranges, like 192.0.2.0/24, used for routing and firewall rules.
CIDR (Classless Inter-Domain Routing) replaced the old class-A/B/C system in 1993 and is how IP address blocks are described today. The notation 192.0.2.0/24 means "the 256 addresses where the first 24 bits match 192.0.2.0," giving the range 192.0.2.0 to 192.0.2.255. A /16 covers 65,536 addresses, a /32 is a single host. CIDR is used everywhere: BGP routing tables, firewall rules, security group definitions, RIR allocations, and VPC subnets. Understanding it is essential when writing allowlists or reading IP intelligence data.