he 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
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 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 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
Classful addressing is obsolete and has not been used in the Internet since the implementation of Classless Inter-Domain Routing (CIDR) starting in 1993.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 class A subnet is not referred to as a class B network. Likewise, a /24 class A or B subnet is not referred to as a class C network. The class is determined by the first three bits of the prefix. In April 2012, IANA allocated 100.64.0.0/10 for use in carrier grade NAT scenarios in RFC 6598.