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)
We will refer to the first block as ‘24-bit block’, the second as ‘20-bit block’, and to the third as ‘16-bit’ block. Note that (in pre-CIDR notation) the first block is nothing but a single class A network number, while the second block is a set of 16 contiguous class B network numbers, and third block is a set of 256 contiguous class C network numbers.
Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, and E. Lear (1996, February), Address Allocation for Private Internets. Retrieved Monday, September 15, 2003, from www.apps.ietf.org/ rfc/ rfc1918.html.