It's not 172.* and 192.*. According to rfc1918:

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

     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)
[download]

-- Randal L. Schwartz, Perl hacker