CIDR block Classless Inter-Domain Routing
The "slash notation" for IP prefixes — 10.0.0.0/24 means the first 24 bits are the network, 8 bits of host = 256 addresses. Replaced the old "class A/B/C" system in 1993.
RFC 1518/1519. Allows variable prefix length (VLSM, Variable Length Subnet Masking) — use /17 if you need 32k addresses, skip wasting a full /16. Also enables route aggregation: a BGP router announces one /20 instead of 16 separate /24s. Common sizes: /24 = 256 addresses, /22 = 1024, /16 = 65536. Cloud VPCs are always specified in CIDR blocks (10.0.0.0/16, 172.16.0.0/12, 192.168.0.0/16 — the RFC 1918 ranges).