Whenever you create a new account with an SIP provider, you’ll get the details necessary to configure your client with that particular entity. Some send these details over email. Others make you log into the service and read the configuration information. A few helpful VoIP providers even list the common SIP clients which are available on the market and tell you what to enter in order to make the connection work properly.
One common detail you’ll probably encounter frequently is the STUN server. It’s in the form of an IP address or a domain name and it’s often not clear where this has to be entered and what its function is. Many SIP clients don’t ask for this information at all and some list it under the “optional” section. Other programs maintain a default STUN server and don’t require any information from you.
But what is a STUN server anyway?
Most of us access the Internet through a gateway. For home connections, this gateway is usually your router. Your router hides the various devices connecting to it and presents a single face to the Internet. No computer on the network behind a router has its own external IP address. Only the router itself has an IP address. Through a process known as Network Address Translation (NAT), the router manager the flow of traffic from the Internet to individual devices behind it.
This is a very useful function and there are many advantages to isolating the devices behind the network to the Internet at large. But sometimes this causes problems for VoIP applications trying to connect two clients resulting in strange behavior such as one way communication. These problems can be solved by using a STUN server.
STUN has quite an imposing expansion - Simple Traversal of User Datagram Protocol (UDP) through Network Address Translators (NAT’s). It’s nothing but a way for a VoIP client to contact an external server and find out what its “real” IP address is and on what port the router is listening to incoming connections. This is a way of “tunneling through” the firewall and can help improve the quality of VoIP services without which the client might not even be able to connect to the provider at all.
Not all clients support STUN functionality. But as VoIP becomes more popular, it’ll probably become a standard feature for both SIP providers and VoIP clients as well. Ask your SIP provider about the configuration requirements for a STUN server and whether you need to enter these details into your SIP client.