Subnetting divides IP networks into smaller segments for security and management. CIDR notation (192.168.1.0/24) shows network bits as a prefix. /24 = 254 usable hosts. /23 = 510 hosts. /16 = 65,534 hosts. Network address (first) and broadcast address (last) are not assignable. Private RFC 1918 ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are not routable on the public internet. Wildcard masks are used in ACL and firewall rule configurations.