Q

Can I connect to my wireless network and wired LAN simultaneously?

Learn how to connect to a wireless network and wired LAN simultaneously in this troubleshooting advice from Lisa Phifer.

We have two offices: Off1 and Off2. I am connected to the network in Off1 via wireless, using an access point with a router. We have a wired LAN in Off2 to which I would also like to connect. We do not have a router in Off2, only a simple 10/100 hub. My wireless connection obtains its IP address and other settings automatically. But, as we do not have a router on our wired LAN, Off2 computers use static IPs.

Other computers in Off2 connect easily through the wired LAN, but as soon as I start my wireless connection, my computer disconnects from the Off2 wired LAN. The wireless LAN becomes confused and continues to disconnect and reconnect.

How can I connect to the Off1 wireless network while simultaneously connect to the Off2 wired LAN?

The symptoms that you describe sound suspiciously like both Off1 and Off2 use the same IP subnet.

If a single device has two connections to the same subnet, one of two things will happen: Either one connection will have... a lower route cost than the other, or routes will be ambiguous and unpredictable. For example, if your wired LAN connection has a lower cost, the wireless connection will always be used whenever it is active. However, if both connections have the same cost, some packets may be routed to the wired LAN and others to the wireless network.

This isn't necessarily a problem unless there is a reason that you want traffic to be routed over one connection and not the other. That's exactly what might be happening if you have two completely disjointed networks that use the same IP address range. For example, suppose your Ethernet connection has the static IP 192.168.1.4 and wants to send traffic to the Off2 server at 192.168.1.5. When your wireless connection isn't active, no problem -- your computer routes the packet through the Ethernet connection. But then you activate your wireless connection and it gets the dynamic IP 192.168.1.100. Your computer may decide to send that packet through the wireless connection, because it is also a valid route to the 192.168.1.0/24 subnet -- possibly even a preferred route. At this point, you won't be able to reach computers on the Off2 wired LAN, even though you are physically connected to it.

For more information on subnetting and IP addressing

View this tip on how to subnet

Test your subnetting skills with this interactive quiz

View our IP addressing and subnetting guide

If this is the cause of your problem, the solution is simple. Just reconfigure the Off1 access point/router to use a different IP subnet and assign dynamic IP addresses from that subnet. For example, if Off2 uses static IPs in the 192.168.1.0/24 subnet, then Off1 can use the 192.168.2.0/24 subnet to avoid overlap. This will eliminate the routing problems described above by eliminating any ambiguity, letting your computer send traffic out the right connection to reach destinations in both networks at once.

However, there is still one further consideration: which connection should be treated as the default route? That is, when your computer tries to send traffic to a destination that's not on either your wireless network or your wired LAN (e.g., a server on the Internet), where should that packet be sent? Because you don't have a router in Off2, the answer is almost certainly the access point/router in Off1. This means that when you statically configure your wired LAN connection, you should not specify a default gateway. When your wireless connection gets an IP address, it should also get a default gateway address -- the local address of the Off1 access point/router.

One final hint: You can always see your computer's routing table by opening a Command Prompt window and typing "route print" -- type "route --h" for instructions on how to use this command.

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This was first published in December 2007

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