Which IP ranges are private, and what happens if I use them on the internet?

RFC 1918 reserves three blocks for private use: 10.0.0.0/8, 172.16.0.0/12 (that's 172.16–172.31 — the /12 catches people out), and 192.168.0.0/16. Internet routers drop these — ISPs filter them at the edge — so a private source address can never receive a reply across the internet. That's exactly why NAT exists: your edge router rewrites private sources to its public address (PAT tracks each flow by port), making replies routable back. Also memorize the lookalikes: 169.254.0.0/16 is APIPA (a host that failed DHCP), 127.0.0.0/8 is loopback, and 100.64.0.0/10 is carrier-grade NAT space. An exam favorite is spotting that 172.32.x.x is public — it's just outside the /12.