Example: I have an IPv4 network, and you have an IPv4 network, and we want to talk to each other. But there's no IPv4 network between us. We need to tunnel our networks somehow through whatever is between us so that we can network to each other. That in between network could be anything, NetBEUI, IPX/SPX, IPv6, whatever. As long as we tunnel through it, our IPv4 networks can see each other (but not the network inbetween).
