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)

Any enterprise that needs globally unique address space is
required to obtain such addresses from an Internet registry. An
enterprise that requests IP addresses for its external connectivity
will never be assigned addresses from the blocks defined above.