I have problem with Visual Studio 2012 (OS: Windows 8.1 Preview)
I found this is the key thing to check, as per (WP8) Emulator (XDE) Troubleshooting Tips on msdn:
Launch the “Network Connections” manager (from the home screen, type in “View Network Connections”, It will be listed under Settings)
Right click on the adapter named: “vEthernet (Internal Ethernet Port Windows Phone Emulator Internal Switch)”
For that adapter, the only items that should be checked under “This connection uses the following items:” should be:
All other items should be unchecked.