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On my desktop computer running Windows 7 64-bit, I have two network ports. The port built into my motherboard is connected directly to my cable modem for internet access. The network connection in Windows for this port is set up to automatically acquire an IP address. The settings acquired look something like:

IP: 192.168.1.x
Subnet: 255.255.255.0
Gateway: 192.168.1.1

The second port is on a Gigabit Ethernet card which connects directly to a second computer via crossover cable. The second computer is only networked via this connection. The network connection on my desktop is configured as:

IP: 192.168.2.1
Subnet: 255.255.255.0
Gateway: 192.168.2.2

And on the second computer, it's configured as

IP: 192.168.2.2
Subnet: 255.255.255.0
Gateway: 192.168.2.1

When I first set the connection up, the two computers were able to communicate fine. My desktop had internet access when I first set this up. No issues at all.

The second computer is usually shut down when not in use. Likewise, the desktop computer is put to sleep when I'm finished with it. The problem is that when my desktop turns on after I take it out of sleep, I lose all internet access. I can ping my router fine, or any other device on the network. If I boot up the second computer, I can RDP into it and ping it fine as well.

So my question is: what causes my dual-network desktop computer to lose internet access when I resume from sleep?

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Why would you configure a gateway IP address that's not a gateway? –  David Schwartz Aug 15 '12 at 4:10
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1 Answer

The "Gateway" setting in IP configuration is telling each machine how to get to networks that they are not connected to, and otherwise don't have a specific route for.

Included in this set of networks is the Internet. You are not directly connected to every network on the internet, and you don't have specific routes for every network on the internet. So your PC gets to those networks by sending these packets to the gateway address, and the gateway should know where to send them.

In your case, you have two gateways defined - one leading to the internet via your cable modem, and another leading to your second PC. Leaving out any methods for working out an ideal path to the internet, your PC with two network cards will randomly pick one of these gateways to send each packet to.

Half would be sent to the internet successfully. The other half would be sent to the other PC which has no idea what to do with them, so sends them right back. The first PC will then either (randomly) send them to the internet gateway, or send them right back to the second PC again, until the packets have hopped back and forth too many times and are discarded.

The solution is straightforward, remove the gateway entry on the 192.168.2.1 network card.

The reason why it works when the other PC is off is that the OS will disregard a gateway that isn't there - as it is unable to send packets to the second PC when it is off, it decides that gateway is broken and uses the cable modem for everything.

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