In Nanyang Technological University’s (NTU) Local Area Network (LAN), whenever one plugs a personal router into a LAN port, the port will be blocked for 10 minutes. If the router is still present after 10 minutes, it gets blocked again. This is probably to ensure that nobody connects a wireless router/access point and turn the non-password secured physical port into a wireless one.
However, this security measure interferes with virtual machines as well. Whenever I start XP Mode, VirtualPC or VirtualBox, I get disconnected from the network. This is probably due to virtualization turning my network card into a Network Address Translation (NAT) device, thus appearing as a router from the outside.
The solution is to connect to NTU Virtual Private Network (VPN) first. Instructions can be found at http://www.ntu.edu.sg/cits/itnetworking/remoteaccess/Pages/quickstartguide.aspx#sslvpn. By doing so, I can access the Internet both from my host and guest OS without getting blocked.