If you've ever wondered how Skype can connect two PC users across the country, both behind firewalls, this article explains it. Here's a taster:

But anyone who has used the popular internet telephony software Skype knows that it works as smoothly behind a NAT firewall as it does if the PC is connected directly to the internet. The reason for this is that the inventors of Skype and similar software have come up with a solution.

Naturally every firewall must also let packets through into the local network - after all the user wants to view websites, read e-mails, etc. The firewall must therefore forward the relevant data packets from outside, to the workstation computer on the LAN. However it only does so, when it is convinced that a packet represents the response to an outgoing data packet. A NAT router therefore keeps tables of which internal computer has communicated with which external computer and which ports the two have used.

The trick used by VoIP software consists of persuading the firewall that a connection has been established, to which it should allocate subsequent incoming data packets. The fact that audio data for VoIP is sent using the connectionless UDP protocol acts to Skype's advantage. In contrast to TCP, which includes additional connection information in each packet, with UDP, a firewall sees only the addresses and ports of the source and destination systems. If, for an incoming UDP packet, these match an NAT table entry, it will pass the packet on to an internal computer with a clear conscience.