Private Network - Private IPv4 Address Spaces

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Classful addressing is obsolete and has not been used in the Internet since the implementation of Classless Inter-Domain Routing (CIDR) starting in 1993.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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