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Building Telephony Systems with OpenSIPS - Second Edition by Bogdan-Andrei Iancu, Flavio E. Goncalves

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Types of NAT

The way in which various NAT devices work is different and varies based on the vendor and models. It is not easy to predict the type of NAT before buying. There are four kinds of NAT devices and they are different in the way they map the internal addresses to external addresses:

  • Full cone
  • Restricted cone
  • Port-restricted cone
  • Symmetric

We will cover each of these devices in the following sections.

Full cone

Full cone devices use a static mapping from the external ip:port pair to the internal ip:port pair. Any external computer can connect to it using the external ip:port pair. This is the case in non-stateful firewalls implemented with the use of filters. The external address is predictable after the first interaction.

A full cone device does ...

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