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!

Searchable, convenient, complete TCP/IP information.
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 Unidirectional (Traditional/Outbound) Operation
Previous Page
Pages in Current Topic/Section
1
23
Next Page
IP NAT Port-Based ("Overloaded") Operation: Network Address Port Translation (NAPT) / Port Address Translation (PAT)
Next Topic/Section

IP NAT Bidirectional (Two-Way/Inbound) Operation
(Page 1 of 3)

Traditional NAT is designed to handle only outbound transactions; clients on the local network initiate requests and devices on the Internet send back responses. However, in some circumstances, we may want to go in the opposite direction. That is, we may want to have a device on the outside network initiate a transaction with one on the inside. To permit this, we need a more capable type of NAT than the traditional version. This enhancement goes by various names, most commonly Bidirectional NAT, Two-Way NAT and Inbound NAT. All of these convey the concept that this kind of NAT allows both the type of transaction we saw in the previous topic and also transactions initiated from the outside network.

The Problem With Inbound NAT: Hidden Addresses

Performing NAT on inbound transactions is more difficult than conventional outbound NAT. To understand why, remember that the network configuration when using NAT is inherently asymmetric: the inside network generally knows the IP addresses of outside devices, since they are public, but the outside network doesn't know the private addresses of the inside network. Even if they did know them, they could never be specified as the target of an IP datagram initiated from outside since they are not routable—there would be no way to get them to the private network's local router.

Why does this matter? Well, consider the case of outbound NAT from device A on the inside network to device B on the outside. The local client, A, always starts the transaction, so device A's NAT router is able to create a mapping between device A's inside local and inside global address during the request. Device B is the recipient of the already-translated datagram, so the fact that device A is using NAT is hidden. Device B responds back and the NAT router does the reverse translation without device B ever even knowing NAT was used for device A.

Now, let's look at the inbound case. Here, device B is trying to send to device A, which is using NAT. Device B can't send to device A's private (inside local) address. It needs device A's inside global address in order to start the ball rolling. However, device A's NAT router isn't proximate to device B. In fact, device B probably doesn’t even know the identity of device A’s NAT router!


Previous Topic/Section
IP NAT Unidirectional (Traditional/Outbound) Operation
Previous Page
Pages in Current Topic/Section
1
23
Next Page
IP NAT Port-Based ("Overloaded") Operation: Network Address Port Translation (NAPT) / Port Address Translation (PAT)
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.