Technology Terms

NAT - (Network Address Translation). It's the technology which allows most people to have more than one computer in their home and still use a single IP address. Most of the time, a router with NAT support gets data packets from the internal network (with internal IP addresses) and sends them to Internet, changing the internal IP address of each packet to the external one.

What's RTP?

RTP stands for Real-Time Transport Protocol. Its purpose is to pass voice data between a caller and the called. The problem is that when you call someone using the RTP protocol, you need to know his IP address and port. This makes RTP quite inconvenient when used alone, since the parts have no way to find one another. This is why people invented SIP.

What's SIP?

SIP (Session Initiation Protocol) looks in syntax like HTTP, human readable text. Its purpose is to help the caller find the IP address and port of the called. It also helps the negotiation of the media types and formats. For example, when you have a PC at home and you want to call your friend from Romania using Free World Dialup (which uses the SIP protocol):