I'm wondering how L2TP works internally to provide transparent IP connectivity. I understand that L2TP tunnels IP packets via UDP port 1721. However I don't see how this provides transparent access. For example let's say I have a VPN connection and my browser connects to google.com. It would normally make a TCP connection to google's IP address over port 80. However something has to intercept this request and route it through L2TP's mechanism.
In the Security Now podcast they did a 4 part series VPNs is great detail. The best part is that there are transcripts online so you don't have to listen to the whole thing if you know what to search for. here's a link to the main page http://www.grc.com/vpn/vpn.htm