NAT Considerations

As with most things, NAT has both advantages and disadvantages. You will want to understand the implications of using NAT on your network. Let's start with the advantages:

• Conservation Using NAT, you save on the number of addresses needed to connect to the Internet. You can use a handful of public addresses to represent thousands of private addresses.

• Flexibility Changing service providers requires only minor changes to your NAT configuration. The only addresses that you need to change are public addresses; NAT allows you to keep your private addressing intact.

• Overlapping networks You can use the same address range in multiple locations and use NAT to connect them.

• Ease IP renumbering As networks expand and you need new addressing schemes, you can roll out the changes gradually. Existing address schemes can remain unchanged.

• Security The actual IP addresses of your hosts are hidden from the outside world. In many instances, the only thing that appears to the Internet is a single IP address doing a lot of surfing.

For the exam, remember all the advantages and disadvantages of using NAT.

Some of the disadvantages of using NAT follow:

• Latency Translating an address takes time. To further complicate things, some protocols also have the address in the payload of the packet, adding more time because two instances of the address need to change.

• Functionality Some applications simply do not work with NAT. Often, NAT cannot find and accurately convert a second address in the payload. Cisco regularly updates NAT's functionality with IOS updates.

• Traceability With multiple NAT translations, it becomes increasingly difficult to trace packets. Sometimes considered a good thing for privacy, it does make troubleshooting your network harder.

• Resource use Translation requires extra CPU cycles to process packets, especially packets with an additional address in the payload. Also, the router consumes additional memory to maintain the translation table.