RFC1918 Networks

Decades ago, when the IP addresses on the Internet were being assigned, multiple blocks were set aside for testing purposes.

Those addresses can be see in the following table:

List of RFC1918 Networks

10.0.0.0 - 10.255.255.255  (10/8 prefix)
172.16.0.0 - 172.31.255.255  (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)