What is Double NAT?

If experiencing one-way audio, periodic phone deregistration, unsuccessful call transfers, error messages on outbound dials, Call Groups not functioning as configured, or periodic dropped calls, the culprit may be Double NAT.

Double NAT occurs when two or more routers on a network are performing Network Address Translation (NAT). Double NAT (sometimes known as double routing) generally does not affect computer use or web browsing but can cause issues with VoIP service.

An example of a simple network with one gateway (say a DSL or Cable modem) provides the gateway a public (WAN) IP address and performs NAT, allowing all computers and devices to share one public IP address.

Computers and devices connected to the gateway are assigned a private IP address. The gateway routes data from and to the public Internet and the computers and devices connected to it on the private network.

When more than one router exists on a network, the Network Address Translation is placed one right after another, creating two private networks (Double NAT).

For help diagnosing Double NAT issues, have a Network Administrator verify the WAN IP of the device to which the phones are connected. The WAN IP should be the public Internet IP address and not a private IP address.

Private IP Address Ranges

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Updated on November 15, 2019

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