CIDR Classless Inter-Domain Routing
Notation for IP networks: 192.168.1.0/24 means the first 24 bits are the network part.
/24 = 256 addresses, /16 = 65k, /32 = a single IP. Replaced the old "class A/B/C" blocks and made fine-grained IP allocation possible. AWS, VPN, firewall — everyone speaks CIDR.