RFC1918

RFC 1918 - Address Allocation for Private Internets - defines standards for IP addresses in a private local network.

Addresses with in these ranges cannot be routed on the Internet:

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)