Set up VPN & NAT reference site

http://chunju.pixnet.net/blog/post/1162502-nat%E5%92%8Cvpn%E6%9C%89%E4%BD%95%E4%B8%8D%E5%90%8C

NAThttp://140.116.177.177/courses/NS_99/pdf/Module10.pdf

VPNhttp://blog.new-studio.org/2010/07/poptop-ubuntu-pptp-vpn.html

http://poptop.sourceforge.net/dox/diagnose-forwarding.phtml?cn=node6&ctni=eth4&ctnia=10.1.3.2&sn=vpnserver&stni=eth12&stnia=10.1.3.1&seni=eth15&senia=10.1.2.1&tenia=10.1.2.2