Private IPv4 Address Spaces
The Internet Engineering Task Force (IETF) has directed the Internet Assigned Numbers Authority (IANA) to reserve the following IPv4 address ranges for private networks, as published in RFC 1918:
| RFC1918 name | IP address range | number of addresses | classful description | largest CIDR block (subnet mask) | host id size | mask bits |
|---|---|---|---|---|---|---|
| 24-bit block | 10.0.0.0 - 10.255.255.255 | 16,777,216 | single class A network | 10.0.0.0/8 (255.0.0.0) | 24 bits | 8 bits |
| 20-bit block | 172.16.0.0 - 172.31.255.255 | 1,048,576 | 16 contiguous class B network | 172.16.0.0/12 (255.240.0.0) | 20 bits | 12 bits |
| 16-bit block | 192.168.0.0 - 192.168.255.255 | 65,536 | 256 contiguous class C network | 192.168.0.0/16 (255.255.0.0) | 16 bits | 16 bits |
Classful addressing is obsolete and has not been used in the Internet since the implementation of Classless Inter-Domain Routing (CIDR) starting in 1993. For example, while 10.0.0.0/8 was a single class A network, it is common for organizations to divide it into smaller /16 or /24 networks. Contrary to a common misconception, a /16 subnet of a class A network is not referred to as a class B network. Likewise, a /24 subnet of a class A or B network is not referred to as a class C network. The class is determined by the first three bits of the prefix.
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