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What Is a STUN Server in VoIP?

Updated on April 12, 2012

VoIP and Network Address Translation

There are a lot of abbreviations used in VoIP. Some of them expand into further abbreviations which can make things very confusing! STUN is one such example – it translates into Simple Traversal of UDP through NATs. Or at least that is what it was before it was redefined into Session Traversal Utilities for NAT. NAT stands for network address translation. Most of us who use the Internet don't connect to an external host directly. Rather, we go through our network which is hooked up to a router or a firewall. This idea serves many purposes, one of which is to protect our local systems from websites or servers which might try and take advantage of an individual PCs insecurity. The router or firewall ensures that all traffic flows through it and doesn't allow an external system to contact an individual node. It does this through a method known as network address translation or NAT.

While this is an excellent technique and is fundamental to the working of networked PCs, it can pose a problem for SIP VoIP protocols when the client and server are unable to communicate properly because the NAT functionality of the firewall is screwing around with the IP addresses. A lot depends on the configuration of the NAT system as well as the capability of the SIP client to support the STUN protocol.

The Working of a STUN server
The Working of a STUN server

Configuring the STUN Server

the STUN protocol is a method for SIP clients to find out what their real IP addresses are. By sending a request to the STUN server, they can obtain this information and make it easier for the SIP call to take place. Sometimes without this functionality, users experience a variety of problems such as one-way audio. Unfortunately, not all SIP clients support STUN functionality and even when they do, it's quite possible for the user who's setting the system up to not understand what it does. This is one of the reasons why automatic configuration and provisioning of VoIP phones is so helpful and useful. It can enable even a completely non-technical person to set up a VoIP phone system.

Sometimes your hosted VoIP services provider who's giving you your SIP accounts will tell you to use their own specific STUN server for resolving IP addresses. It's good to follow their advice in such circumstances, but there are plenty of public STUN servers will do the same for you for free. If you're setting up an SIP client all by yourself and you notice strange issues like one-way audio, keep in mind that the STUN server may need to be set. Contact a VoIP provider like OnSIP to find out how you can get started today.


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    • verizon-hurts-sip profile image

      verizon-hurts-sip 5 years ago

      the truly unfortunate have an ISP that is "SIP hostile"

      this requires a letter writing campaign to the FCC and possibly a class action lawsuit against:



      time warner