The Internet Assigned Numbers Authority (IANA) has reserved several blocks of IP addresses that an organization may assign for its own private internet. These blocks are defined in RFC 1918:
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 (10.0.0.0/8 prefix)
- 172.16.0.0 - 172.31.255.255 (172.16.0.0/12 prefix)
- 192.168.0.0 - 192.168.255.255 (192.168.0.0/16 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.
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