| Range | CIDR |
|---|---|
| 10.0.0.0 - 10.255.255.255 | 10.0.0.0/8 |
| 172.16.0.0 - 172.31.255.255 | 172.16.0.0/12 |
| 192.168.0.0 - 192.168.255.255 | 192.168.0.0/16 |
Private addresses are not routed on the Internet; NAT (Network Address Translation) on the router translates them to a public address. APIPA addresses 169.254.x.x appear when a client fails to obtain a DHCP address.
Why private addresses exist
There are only about 4.3 billion IPv4 addresses. Private ranges (RFC 1918) let every home and office reuse the same addresses (192.168.1.1 appears in millions of networks) without clashing on the Internet, because those packets are not forwarded by Internet routers.
What NAT does
Network Address Translation on the edge router rewrites private source addresses to the router's public address when traffic goes out, and rewrites replies back again. From the Internet's point of view, many internal PCs share one (or a few) public IP(s). From the LAN's point of view, each PC still has its own private address.
| Direction | What happens |
|---|---|
| Outbound from PC 192.168.1.50 | Router replaces source with public IP (and tracks the port mapping) |
| Reply from a web server | Router looks up the mapping and sends the packet to 192.168.1.50 |
| Unsolicited inbound to a private IP | Blocked unless you create a port-forward / firewall rule |
APIPA (169.254.x.x)
If a client is set to use DHCP but cannot reach a DHCP server, Windows and some other systems assign themselves an address in 169.254.0.0/16. That lets devices on the same cable talk to each other locally, but they usually cannot reach the Internet or other subnets. Seeing 169.254... in ipconfig is a strong hint that DHCP failed (cable unplugged, wrong VLAN, or DHCP server down).
Everyday example: your home Wi-Fi uses 192.168.x.x. The ISP gives your router one public address. Every phone and laptop shares that public address through NAT when browsing.
Remember
- Private addresses stay inside the organisation; NAT translates them for the Internet.
10/8,172.16/12and192.168/16are the three private ranges.169.254.x.xusually means "DHCP failed".