For setting up private networks, three IP address ranges have been reserved by IANA:

Reserved IP address range Network class
10.0.0.0 - 10.255.255.255 Class A
172.16.0.0 - 172.31.255.255 Class B
192.168.0.0 - 192.168.255.255 Class C

These private IP address ranges have been documented in  RFC 1597 and RFC 1918.

Private IP address ranges are not routed in the Internet and can be used without registration in any number of private networks.