A method for dealing with the problem that no more worldwide unique Class A IP addresses and very few Class B addresses are available for new networks. Lots of large companies need lots of addresses, but large blocks of addresses are not available.
Most companies use a firewall between the Internet and their internal network.
Each user does not need their own external IP address, since the connections are initiated by the users, who need only a unique socket number , not IP address. Internally, the organization uses any IP addresses it wishes. RFC 1597 reserves some IP addresses for this private internal use. Since these particular addresses can never be valid Internet addresses, routers know whether to do a translation or not. These reserved addresses are listed in the following table.