Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

The whole site in one document for easy reference!
The TCP/IP Guide

Custom Search







Table Of Contents  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

Previous Topic/Section
IP NAT Port-Based ("Overloaded") Operation: Network Address Port Translation (NAPT) / Port Address Translation (PAT)
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
IP NAT Compatibility Issues and Special Handling Requirements
Next Topic/Section

IP NAT "Overlapping" / "Twice NAT" Operation
(Page 2 of 3)

Dealing With Overlapping Blocks By Using NAT Twice

The solution to this dilemma is to use a more sophisticated form of NAT. The other versions we have seen so far always translate either the source address or the destination address as a datagram passes from the inside network to the outside network or vice versa. To cope with overlapping addresses, we must translate both the source address and the destination address on each transition from the inside to the outside or the other direction. This technique is called Overlapping NAT in reference to the problem it solves, or “Twice NAT” due to how it solves it. (Incidentally, despite the latter name, regular NAT is not called “Once NAT”.)

Twice NAT functions by creating a set of mappings not only for the private network the NAT router serves, but also for the overlapping network (or networks) that conflict with the inside network's address space. In order for this to function, Twice NAT relies on the use of the TCP/IP Domain Name System (DNS), just like bidirectional NAT. This lets the inside network send requests to the overlapping network in a way that can be uniquely identified. Otherwise, the router can't tell what overlapping network our inside network is trying to contact.


Previous Topic/Section
IP NAT Port-Based ("Overloaded") Operation: Network Address Port Translation (NAPT) / Port Address Translation (PAT)
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
IP NAT Compatibility Issues and Special Handling Requirements
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.