Early network design, when global end-to-end connectivity was envisioned for communications with all Internet hosts, intended that IP addresses be uniquely assigned to a particular computer or device. However, it was found that this was not always necessary as private networks developed and public address space needed to be conserved (IPv4 address exhaustion).
Computers not connected to the Internet, such as factory machines that communicate only with each other via TCP/IP, need not have globally-unique IP addresses. Three ranges of IPv4 addresses for private networks, one range for each class (A, B, C), were reserved in RFC 1918. These addresses are not routed on the Internet and thus their use need not be coordinated with an IP address registry.
Today, when needed, such private networks typically connect to the Internet through network address translation (NAT).
| IANA-reserved private IPv4 network ranges | |||
|---|---|---|---|
| Start | End | No. of addresses | |
| 24-bit Block (/8 prefix, 1 x A) | 10.0.0.0 | 10.255.255.255 | 16,777,216 |
| 20-bit Block (/12 prefix, 16 x B) | 172.16.0.0 | 172.31.255.255 | 1,048,576 |
| 16-bit Block (/16 prefix, 256 x C) | 192.168.0.0 | 192.168.255.255 | 65,536 |
Any user may use any of the reserved blocks. Typically, a 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).
IPv4 address depletion
The IP version 4 address space is rapidly nearing exhaustion of available, officially assignable address blocks.
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