You cannot make more than one client connection over a NAT device to Windows File Server

When two client computers try to use the server message block (SMB) protocol to connect to the same server across a network address translation (NAT) device, the more recent client connection may reset the earlier client connection. If a client and a server that use the SMB protocol over a NAT device are copying files, that session may be reset when another client uses the SMB protocol over the same NAT device to the same server.

When a new connection is established to a server, the server checks for previous connections from the same client IP address and deletes any previous connections. Because NAT acts as a proxy for the SMB protocol by using the same IP address from the two client connection requests, the earlier connection is terminated.

This fix applies only to the SMB protocol traffic over TCP NetBIOS port 139. The connectivity problem that is mentioned in this article still occurs on client computers that are running Windows 2000 or later if you make the connection with direct hosting on port 445. By default, Windows 2000 uses this port.

You can establish no more than one direct hosting client connection across a NAT device. If you must have more than one simultaneous SMB protocol, disable either direct hosting on the Windows 2000 file server or on all clients behind the NAT device.

To disable direct hosting on the server:

1. Start Registry Editor.


2. Locate and then click the following registry key:
   
   HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NetBT\Parameters


3. Add the following registry value:
   
   Value Name: SmbDeviceEnabled
   Type: REG_DWORD
   Value Data: 0
   
   The default setting is 1 (enabled)

When you complete these steps, clients must use port 139 NetBIOS over TCP/IP to make a connection.

http://support.microsoft.com/?scid=kb%3Ben-us%3B301673&x=8&y=7

You cannot make more than one client connection over a NAT device to Windows File Server