Ethernet Shield preventing internet access

Hi, I have a problem that is driving me mad. I'm using an Arduino Uno and W5100 ethernet shield connected via lan to a laptop with a wireless internet connection. When the Ethernet shield is connected the laptop can no longer access the internet. The arduino is running the webserver program located in the examples folder. Any help would be greatly appriciated.

Perhaps you have inadvertently assigned the same IP address to the Arduino and the laptop? Try a different IP address in the sketch.


Also insure your subnets are different on the ethernet interface and the wireless interface. If they are the same, that causes routing problems in the laptop.

When the Ethernet shield is connected the laptop can no longer access the internet.

Just what components are in your "lan"? If you are using a wireless router, can you still log into the router from the laptop?

Thanks everybody. I've tried changing the subnet mask on the LAN to and and I can now access the internet with the ethernet shield plugged in but cannot access the ethernet sheild. Laptop lan address shield address

Maybe I was not clear. Leave the subnet mask set at The subnets need to be different.

Check your wireless interface after you connect without the shield connected to the laptop. What ip does it have? If it is 192.168.1.x (x is any number from 2 to 254), then you should change the ip assignment on the laptop ethernet interface to (or

Edit: I would use on the ethernet interface if the wireless interface isn't using that subnet. If it is a Windows OS laptop, you can use internet connection sharing to connect to the internet with the shield, or any other device connected to the ethernet port. My customers are normally connecting game consoles this way.

Google search "Windows internet connection sharing" for details and instructions.

Edit2: The reason it works the way it does when you change the subnet mask to a larger subnet on the ethernet interface is: The interface with the smallest subnet is selected first. It is a routing thing.

Tim, Many thanks for your help, its working perfectly now. I'd just better clarify what I did to get this working so that anybody having the same problem knows how to sort it out. My wireless ip address was in the form 192.168.1.x. I changed the lan ip address to and set the arduino shield address to, hay presto it works !!!. I'm guessing the subnets have to be the same for the lan port and the arduino. Again many thanks.