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Most VPNs rely on tunneling to create a private network that reaches across the Internet. Essentially, tunneling is the process of placing an entire packet within another packet and sending it over a network. The protocol of the outer packet is understood by the network and both points, called tunnel interfaces, where the packet enters and exits the network. Tunneling requires three different protocols: * Carrier protocol - The protocol used by the network that the information is traveling over * Encapsulating protocol - The protocol (GRE, IPSec, L2F, PPTP, L2TP) that is wrapped around the original data * Passenger protocol - The original data (IPX, NetBeui, IP) being carried Tunneling has amazing implications for VPNs. For example, you can place a packet that uses a protocol not supported on the Internet (such as NetBeui) inside an IP packet and send it safely over the Internet. or you could put a packet that uses a private (non-routable) IP address inside a packet that uses a globally unique IP address to extend a private network over the Internet.