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. ...