As the Internet grows and companies need more and more IP addresses, the number of available
IP addresses diminishes. This is one of the main reasons for the implementation of NAT and
PAT—technologies that are critical to overcoming the shortage of IP addresses. You need to
understand how NAT and PAT operate and how to configure each of them.
These two protocols, which allow for specifically defined address translations, provide some
other interesting uses as well. For instance, NAT and PAT enable private IP networks to use
unregistered IP addresses to access outside networks such as the Internet. They also provide the
ability to reuse assigned IP addresses already in use on the Internet. In addition, they appropriately
translate the addresses in two merged intranets, such as those of two merged companies.
Finally, NAT and PAT translate internal IP addresses assigned by an old Internet service provider
(ISP) to a new ISP’s newly assigned addresses without manual configuration of all the local
network interfaces.
There are some disadvantages to using NAT and PAT in a network. Specifically, they don’t
allow for a full end-to-end communication between two hosts. Some protocols carry IP
address information in the payload of the packet that might not get translated by the border
NAT router.
There are many IOS commands used specifically for troubleshooting NAT problems. The
show ip nat translations command is one of the most useful, in addition to the debug ip
nat feature. PAT also has its own troubleshooting commands; the show ip pat command is
the most important.