Private IP Addresses Range
While Internet hosts require a globally unique IP address, private hosts that are not connected to the Internet can use any valid address, as long as it is unique within the private network. But because many private networks exist alongside public networks, grabbing “just any address” is strongly discouraged.
In 1994, the IETF released a document titled RFC 1597, which stated that many organizations used TCP/IP and IP addresses, yet remained unconnected to the Internet. RFC 1597 was updated to RFC 1918, suggesting that a block of the available IP address space could be set aside for private networks. Private networks that needed IP for application support without requiring connectivity to the Internet could simply use addresses from those allocated for private use.
Three blocks of IP addresses (one Class A network, 16 Class B networks, and 256 Class C networks) have been designated for private, internal use. Addresses in this range are not routed on the Internet backbone (see figure). Internet routers are configured to discard private addresses.
When addressing a nonpublic intranet, these private addresses can be used instead of globally unique addresses. When a network using private addresses must connect to the Internet, it is necessary to translate the private addresses to public addresses. This translation process is NAT. A router is often the network device that performs NAT.