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Machines not connected to the outside world (e.g. factory machines that communicate with each other via TCP/IP) need not have globally-unique IP addresses. Three ranges of IPv4 addresses for private networks, one per class, were standardized by RFC 1918; these addresses will not be routed, and thus need not be coordinated with any IP address registrars.
IANA Reserved Private Network Ranges Start of range End of range
The 24-bit Block 10.0.0.0 10.255.255.255
The 20-bit Block 172.16.0.0 172.31.255.255
The 16-bit Block 192.168.0.0 192.168.255.255
Each block is not necessarily one single network, although it is possible. Typically the network administrator will divide a block into subnets; for example, many home routers automatically use a default address range of 192.168.0.0 - 192.168.0.255 (192.168.0.0/24).