This is a quote from "RFC 1918", Address Allocation for Private Internets February 1996:

3. Private Address Space
The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for private internets:

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)

The full text of rfc1918