Tip

Network address translators

What do you do if your ISP changes, and you have all these fixed IP addresses assigned to your network users? One thing you can do is reassign IP addresses to everyone, by going to the workstation and resetting them. Another is network address translation, the subject of this tip, excerpted from

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Managing IP Networks with Cisco Routers, by Scott M. Ballew, published by O'Reilly & Associates Inc.


One way to avoid re-addressing your network hosts each time you change Internet providers is to use private IP networks from EFC 1918 internally, and then install a device known as a Network Address Translator (NAT) between your network and your Internet connection. A NAT sits between an organization's network and the Internet, or between two organization's networks, and translates IP addresses from private internal addresses to globally unique external addresses.

The idea behind NAT is that a few of your internal hosts are accessing the Internet at any given time, so they can share a small pool of dynamically allocated addresses. When a host needs to access the Internet, the NAT allocates an address for it. When packets from the host are sent to the Internet, the NAT replaces the internal address with this dynamically allocated external address. When packets come back for that address, the NAT reverses the substitution and replaces the external address with the corresponding internal address. When the host is done, the NAT releases the external address for use by another host. NATs are not without their problems, though. The biggest problem is that they can only replace addresses in the IP header and certain other well-known locations in the data portion of the packets. For example, in the FTP protocol, one machine tells the other the IP address and port on which it is listening for a data connection. This information must be corrected by the NAT or the file transfer will fail. A NAT can handle this situation for common protocols like FTP, but may not be able to handle newer protocols or protocols that aren't widely used.

NAT technology is fairly new, and may not be readily available in the performance or price you want. At this time, only a few commercial products exist, but more are likely to appear as address space becomes harder to obtain. Bu being aware of NAT technology, you can prepare your organization to take advantage of it as it becomes more widespread. As a result, your network will be more flexible and ready to adapt.


To learn more about Managing IP Networks with Cisco Routers, or to buy this book, click here to go to our online bookstore.


This was first published in June 2002

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