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

Question

I know 192.168.x.x is "private" but I keep seeing 10.x and 172.something too. What exactly are the ranges, and why do they need NAT?

NetForge Team · Accepted Answer

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.

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

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