Port forwarding (also known as port mapping) allows computers to connect to a specific device within a private network. It is extremely useful if you are attempting to use VoIP devices in an environment that uses Network Address Translation (NAT), are using the NAT proxy yet are still encountering issues such as registration failing and call issues.
If you are unsure what NAT is, it may be helpful to read this guide and then return to read the rest of this knowledgebase article.
In order to help explain how port forwarding works, it a good idea to understand what ports are first.
A brief guide to ports
An IP address is made up of 65535 individual ports. When information is sent to an IP address a specific port is also used, which allows one IP address to send and receive data from multiple applications at the same time. Without ports, you would be limited to doing one thing on the Internet at a time.
When a port is in use, no other application can use it.
Using port forwarding to send data straight to one device
Port forwarding allows you to choose to send data received on a port to a specific device in your private network. Think of it like a bypass that has been built straight from one point (your public IP that sits in front of your router) to another (a device behind NAT).
Why would you need to do this? As explained in this guide, VoIP calls are affected by NAT because a different port is used for the transmission of audio packets than was used to set up the call. This can often lead to hearing no audio from the other party when you make or receive a VoIP call (the incoming audio packets are not sent on to your phone because the router has no idea where to send them).
Port forwarding allows you to specify that all communications received on a certain port are forwarded directly to your VoIP phone. You set port forwarding rules up on your router, entering details of your phone and the ports you wish to "reserve" only to be used by this device. This ensures that no other devices in your private network are able to use this port.
You then have to also ensure that you tell your phone to tell other devices to send inbound audio via this port. This will then be done automatically when your call is being setup.
Setting up port forwarding
For help setting up port forwarding, please use this guide.
Setting up multiple VoIP phones
Setting up multiple VoIP phones in a private network is more complicated, because you need to specify different ports for each different phone. If you have read the whole of this article the reason should hopefully be obvious - a port can only be used by one application at a time, and when you set up port forwarding all traffic received on that port is sent straight to one device.