Double NAT

What is Double NAT (Network Address Translation)?

Double NAT within the context of VoIP traffic indicates that multiple local networks are handling NAT before getting to the device that is attempting to be contacted. Generally, Double NAT is caused by multiple routers on a network. Gateways that are in passthrough mode will not cause Double NAT. Layer 3 managed switches have the ability to add static routes that could act like NAT, but these routers are not set up like this by default.

Double NAT causes intermittent issues that include:
 

• One-way audio
• Phone Deregistration
• Unsuccessful transfers
• Error messages when dialing a phone number
• Call Groups not ringing properly
• Dropped Calls

Routers and Gateways are networking devices that perform NAT, which help route incoming Internet traffic to the correct device on the local network. Nextiva recommends a Network Administrator ensures the WAN IP of the local network is an IP address that falls outside the private address ranges in the ranges below:
 

• 10.0.0.0 - 10.255.255.255
• 172.16.0.0 - 172.31.255.255
• 192.168.0.0 - 192.168.255.255

Why does Double NAT cause problems?

Double NAT usually causes intermittent issues with VoIP traffic.  When there is little traffic on the network, both routers will use the same ports to relay data.  When this occurs, there are usually no interruptions with VoIP service as shown below:




When the network gets busy, one router may change the port it uses, causing issues with relaying traffic to and from the Nextiva servers as shown below:

In this example, because the port was changed on Router 2, the Nextiva server will reject the incoming data as the port no longer matches what it sent out on causing the phone to drop its call or lose registration.