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Private addressing is the single biggest reason the internet didn't run out of IPv4 addresses in 2005. Every device on your home network — laptop, phone, TV, thermostat — has a private IP behind one public IP. Millions of networks reuse the same ranges without conflict.
| Range | CIDR | Size | Common use |
|---|---|---|---|
| 10.0.0.0 – 10.255.255.255 | 10.0.0.0/8 | ~16.7M addresses | Large enterprises, cloud VPCs |
| 172.16.0.0 – 172.31.255.255 | 172.16.0.0/12 | ~1M addresses | Mid-size networks |
| 192.168.0.0 – 192.168.255.255 | 192.168.0.0/16 | ~65K addresses | Home and small office |
These ranges are never routed on the public internet. ISPs drop them at their edge. If you `traceroute` to 10.0.0.1 from home, you'll see the response come from your own router or a local ISP cache — never from a real 10.x host.
APIPA deserves its own sentence. When a Windows or Linux host can't reach a DHCP server, it self-assigns from 169.254.0.0/16. If you ever see a client with a 169.254.x.x address, you know immediately the DHCP request never landed — the network layer is fine, but something is broken between the client and the DHCP server.
That `169.254.87.42` is a red flag on any network. Either DHCP isn't running, the client can't reach it, or the DHCP scope is exhausted. The `valid_lft` counter counts down toward the next DHCP retry — a healthy host would have a normal RFC 1918 or public address here.
Network Address Translation rewrites the source IP (and often port) of packets leaving a private network so they appear to come from a public address. Your home router does this in hardware. Your enterprise firewall does it in software at much higher scale.
NAT isn't free. It breaks end-to-end connectivity, complicates peer-to-peer protocols, and makes log correlation harder — you see the router's public IP in server logs, not the user's private one. Section 13 covers the mechanics in detail; for now, know that NAT is the reason private addressing scaled.
Do not use RFC 1918 ranges on any device that will be reachable from the public internet without NAT in front. Overlapping private addressing between two merged companies is one of the most painful network refactors you'll ever do — pick your ranges carefully, and document them.
Run `ip -4 addr` on every device you own. Note the public vs. private split, and trace your home router's NAT behavior by visiting `ifconfig.me` from two different devices behind the same router. You'll see the same public IP twice.
Finish the lesson once you have worked through the material. This awards ★ 40 XP.