RFC 1918 specifies private IP address ranges which are not routable on the public internet.
The idea originally was to assign IP addresses for hosts that could communicate with each other inside a network, but without access to the public internet (remember that public IPs cost money).
There are a few RFC 1918 address ranges:
| Start | End | Subnet mask |
|---|---|---|
10.0.0.0 | 10.255.255.255 | /8 |
172.16.0.0 | 172.31.255.255 | /12 |
192.168.0.0 | 192.168.255.255 | /16 |