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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  IP Network Address Translation (NAT) Protocol

IP Network Address Translation (NAT) Protocol 1 2 3 4 5 IP NAT Address Terminology

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 like IPSec 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.

IP Network Address Translation (NAT) Protocol 1 2 3 4 5 IP NAT Address Terminology