Network Address Translation
Since 1992, CIDR has not been the only mechanism directly involved in slowing down the IPv4 address shortage. Over the years, the NAT mechanism (defined in RFC 1631), seen as a short-term solution, played a key role by allowing organizations to use few Internet globally unique unicast IP addresses for their large networks. NAT typically translates packets from a network, which uses globally unique unicast IP addresses or a private address space as defined by RFC 1918, to the Internet.
IANA has reserved three blocks of IP addresses for private addressing. Address spaces 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 are used for address translation with the Internet.
Figure 1-4 shows networks using private addressing. ...