I am having trouble with having Wi-Fi and LAN connections on the same computer. I have a LAN that isn’t connected to the Internet along with Wi-Fi access to the Internet. When using XP, I used to be able to connect to my LAN to access my network hard drive while connected to the Internet by Wi-Fi. Now, when using Windows 7, I can’t connect to the Internet while the LAN is connected. I have to switch back and forth. Can you help with attaining simultaneous network connections?
You should be able to have simultaneous network connections to two separate networks in Windows XP or Windows 7. The problem you are experiencing may be accidental or intentional.
If your two connections are in different subnets, each should serve as the route to any destination on that subnet, but only one will serve as your computer's default gateway for all other destinations (such as the Internet). For example, consider the following response to "netstat –r."
0x1 ........................... MS TCP Loopback interface
0x2 ...00 d0 b7 12 16 fd ...... Intel(R) PRO/100 S Management Adapter
0x10004 ...00 14 d1 6a 70 51 ...... TRENDnet Wireless N speed USB Adapter
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 10.0.0.1 10.0.0.70 2
0.0.0.0 0.0.0.0 192.168.1.1 192.168.1.2 25
10.0.0.0 255.255.255.0 10.0.0.1 10.0.0.1 2
10.0.0.170 255.255.255.255 127.0.0.1 127.0.0.1 2
10.255.255.255 255.255.255.255 10.0.0.1 10.0.0.1 2
127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
192.168.1.0 255.255.255.0 192.168.1.2 192.168.1.2 25
192.168.1.2 255.255.255.255 127.0.0.1 127.0.0.1 25
192.168.1.255 255.255.255.255 192.168.1.2 192.168.1.2 25
Default Gateway: 10.0.0.1
In this example, an Intel LAN connection is assigned IP address 192.168.1.2, while a TRENDnet Wi-Fi connection is assigned IP address 10.0.0.70. Anything sent to any destination on 10.0.0.0 will use the Wi-Fi connection, while anything sent to any destination on 192.168.1.0 will use the LAN connection. Furthermore, everything sent to any other destination will be routed through the default gateway 10.0.0.1, which happens to be reached using the Wi-Fi connection.
One common error occurs when both Wi-Fi and LAN connections receive addresses from the same subnet. In that case, the "faster" connection will have a smaller metric, causing it to be used to send all traffic. The slower connection may not be used at all. If some destinations in the same subnet are physically reached only through that slower connection, they will not be reachable so long as the faster connection is active. Use netstat -r to see if this is your problem. If so, fix it by assigning a different subnet to your LAN connection and LAN-connected hard drive.
Another possibility is that your computer is intentionally configured to prevent simultaneous network connections to more than one network. Employers sometimes enforce this policy for security reasons, so that a laptop connected to an outsider via Wi-Fi cannot become a "backdoor" point of entry into the corporate LAN. Here again, you may be able to diagnose this by looking at netstat, before and after connecting to Wi-Fi. If you suspect this on a corporate-issued laptop, talk to your IT administrator.
This was first published in November 2011