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What is NAT?

Article ID: 000065
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What is NAT?

NAT (Network Address Translation) is a technology most commonly used by firewalls and routers to allow multiple devices on a LAN with 'private' IP addresses to share a single public IP address.

A private IP address is an address, which can only be addressed from within the LAN, but not from the Internet outside the LAN.

When we have to make a call from your phone on your private network to the public internet, we call this "NAT Traversal".

Why is it a problem for VoIP?

Firstly, the SIP protocol is badly designed. It was not designed with NAT traversal in mind, and has to be 'fudged' to make it work properly. A key problem is that the SIP protocol only deals with call setup and signalling.

The voice traffic is handled by a separate protocol (RTP) and uses a randomly negotiated port. This means that your router often sees random packets arriving, without knowing which internal device they are destined for.

At first, for both the calling and the called party everything will appear just fine. The called party will see the calling party's Caller ID and the telephone will ring while the calling party will hear a ringing feedback tone at the other end. When the called party picks up the telephone, both the ringing and the associated ringing feedback tone at the other end will stop as one would expect. However, the calling party will not hear the called party (one way audio) and the called party may not hear the calling party either (no audio).

How do we work round NAT problems?

Firstly, the best way to work round it is to avoid it, and to use a broadband service which provides public IPs. Most of our big voip users have done this - a public ip upgrade to your internet service is typically not expensive.

However, it is not practical to mandate that everyone changes their broadband service - 99% of people use NAT.

Therefore, we use a system called a "Peerpoint Centrex" from Jasomi ( which is a proprietary black box that attempts to bring some order to SIP and RTP packets, so they work on predicable ports and are sent in a firewall friendly manner. This works for most situations.

How can I remove NAT from my network?

We strongly advise that all customers who are using VoIP for their core business place their phones outside of any ADSL firewall, on an internet connection where they have public IP addresses.

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