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IP NAT Overview, Motivation, Advantages and Disadvantages
(Page 5 of 5)
Disadvantages of NAT
The above are all good reasons to
use NAT, but there are drawbacks to the technique as well. Some of these
take away part of the benefit in certain items in the list above:
- Complexity: NAT represents one more complexity
in setting up and managing the network. It also makes troubleshooting
more confusing due to address substitutions.
- Problems Due to Lack of Public Addresses:
Certain functions won't work properly due to lack of a real
IP address in the client host machines.
- Compatibility Problems With Certain Applications:
I said above that NAT was only mostly transparent. There
are in fact compatibility issues with certain applications that arise
because NAT tinkers with the IP header fields in datagrams
but not in the application data. This means tools like FTP,
which pass IP addresses and port numbers in commands, must be specially
handled, and some applications may not work.
- Problems With Security Protocols: Protocols
are designed to detect modifications to headers and commonly balk at
the changes that NAT makes, since they cannot differentiate those changes
from malicious datagram hacking. It is still possible to
combine NAT and IPSec, but this becomes more complicated.
- Poor Support for Client Access: The lack
of a public IP address for each client is a double-edged sword; it protects
against hackers trying to access a host but also makes it difficult
for legitimate access to clients on the local network. Peer-to-peer
applications are harder to set up, and something like an organizational
web site (accessed from the Internet as a whole) usually needs to be
set up without NAT.
- Performance Reduction: Each time a datagram
transitions between the private network and the Internet, an address
translation is required. In addition, other work must be done as well,
such as recalculating header checksums. Each individual translation
takes little effort but when you add it up, you are giving up some performance.
Many organizations feel that the
advantages outweigh the disadvantages, especially if they do use the
Internet in primarily a client/server
fashion, as most do. For this reason NAT has become quite popular. One
should always bear in mind, however, that the main problem that led
to NAT is lack of address space. IPv6 fixes this problem, while NAT
merely finds a clever workaround for it. For this reason,
many people consider NAT a kludge. Once IPv6 is deployed,
it will no longer be needed, and some folks don't like it even for IPv4.
On the other hand, some feel its other benefits make it worthy of consideration
even in IPv6.
Note: A kludge (or kluge) is something that is used to address a problem in an inelegant way, like hammering a nail using the side of an adjustable wrench. (I would never do such a thing, of course not
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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