others through the Internet can be a particularly onerous restriction. The solution: private
addressing with the use of Network Address Translation (NAT).
allows a host that does not have a valid registered IP address to communicate with other hosts
through the Internet. The hosts may be using private addresses or addresses assigned to another
organization; in either case, NAT allows these addresses that are not Internet-ready to continue
to be used but still allow communication with hosts across the Internet.
invalid address to the rest of the Internet. The NAT function changes the IP addresses as
necessary inside each IP packet, as shown in Figure 5-25.
the destination address is changed each time a packet is forwarded back into the private
network. Network 200.1.1.0 has been registered as a network owned by Cisco in Figure 5-25,
with address 200.1.1.1 configured as part of the NAT configuration. The NAT feature,
configured in the router labeled NAT, performs the translation. As you might expect, NAT
certainly requires more processing than simply routing the packet. Cisco does not recommend
using NAT for a large volume of different hosts.
instead using a network number registered to another company. (A client company of mine had
originally done just that--ironically, the company was using a network number registered to
Cabletron, which my client saw used in a presentation by an ex-Cabletron employee who then
worked at 3COM. The 3COM SE explained IP addressing using the Cabletron registered
network number; my client liked the design and took him at his word--exactly.) If one company