RFC 1918
1996 specification that reserves three IPv4 blocks for private use: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. Never routes on the internet — must be NATed before packets leave the LAN.
Home routers default to 192.168.0.0/24 or 192.168.1.0/24. Enterprise networks typically 10.0.0.0/8 for plenty of flexibility. 172.16-31.x is used less, but exists. CGNAT adds 100.64.0.0/10 on top (RFC 6598). IPv6 equivalent: ULA (Unique Local Address) in the fc00::/7 space. Classic merger headache: two companies merge, both use 10.0.0.0/8 → IP collisions, must be renumbered. Cloud VPCs: AWS/GCP default to locally routable RFC 1918 IPs, but you can choose your own ranges.