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, devices not ringing that should be, 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.

One of the more common causes of this is if you have a Modem/Router unit from your ISP and a second Router plugged into that. Most often you will need to contact your ISP and have them enable Bridge mode on their device. This will disable the router function of their device and leave everything up to the other hardware that is plugged in.  Sometimes updates that are pushed out by the ISP will change these settings and disable bridge mode. If/when this happens, you will most likely need to contact the ISP again to have the Bridge Mode settings turned back on.

You can also enable Bridge Mode on the additional router that is plugged in, while leaving it disabled on the ISP modem if you prefer.