ftp://ftp.rfc-editor.org/in-notes/rfc1597.txt

Private Address Space

   The Internet Assigned Numbers Authority (IANA) has reserved the
   following three blocks of the IP address space for private networks:

# Class-A-Netze: 10.0.0.0 - 10.255.255.255
# Class-B-Netze: 172.16.0.0 - 172.31.255.255
# Class-C-Netze: 192.168.0.0 - 192.168.255.255