By default, Windows Vista and the Windows Server 2008 operating system do not support Internet Protocol security (IPsec) network address translation (NAT) Traversal (NAT-T) security associations to servers that are located behind a NAT device. Therefore, if the virtual private network (VPN) server is behind a NAT device, a Windows Vista-based VPN client computer or a Windows Server 2008-based VPN client computer cannot make a Layer Two Tunneling Protocol (L2TP)/IPsec connection to the VPN server. This scenario includes VPN servers that are running Windows Server 2008 and Microsoft Windows Server 2003. 

Because of the way in which NAT devices translate network traffic, you may experience unexpected results when you put a server behind a NAT device and then use an IPsec NAT-T environment. Therefore, if you must have IPsec for communication, we recommend that you use public IP addresses for all servers that you can connect to from the Internet. However, if you have to put a server behind a NAT device and then use an IPsec NAT-T environment, you can enable communication by changing a registry value on the VPN client computer and the VPN server. 

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IPSec

AssumeUDPEncapsulationContextOnSendRule (DWORD (32-bit) Value)

• 0 - A value of 0 (zero) configures Windows so that it cannot establish security associations with servers that are located behind NAT devices. This is the default value.
• 1 - A value of 1 configures Windows so that it can establish security associations with servers that are located behind NAT devices.
• 2 - A value of 2 configures Windows so that it can establish security associations when both the server and the Windows Vista-based or Windows Server 2008-based VPN client computer are behind NAT devices.

This key may also need to be set on L2TP/IPSec VPN clients who connect to this server if connecting from bethind NAT-T.