CIDR (Classless Inter-Domain Routing)
Also known as: classless inter-domain routing, slash notation
The notation and allocation scheme that writes IP ranges as address/mask (e.g. 10.0.0.0/8), replacing the old class A/B/C system.
CIDR replaced the rigid class A/B/C address scheme with variable-length masks, letting address space be allocated and routed in blocks of any power-of-two size. A /24 is 256 addresses, a /23 is 512, and so on.
CIDR notation is used everywhere in networking: RIR allocations, BGP announcements, firewall rules, and geofeeds. Aggregating adjacent prefixes into a shorter CIDR block (supernetting) keeps the routing table smaller.
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