IETF RFC 1918 establishes three blocks of IPv4 addresses for free unlimited use on private networks. There are blocks from each of Classes A, B, and C. These reserved address blocks are as follows:
Because RFC 1918 addresses are reserved for private use, they are not routable on public networks.
RFC 1918 has played a very important role of extending the life of IPv4 and delaying the introduction of IPv6.
Network address translation (NAT) often is used to deploy RFC 1918 addresses on the inside network, reserving as little as a single public IP address for the outside facing network interface.