Hello friends,
Let me apologize if this question does not belong here.
I have two boxes with Intel 1000 GT NIC in each of them connected to an unmanaged switch at home. Whenever I turned on the second box, the network connection to the first box would drop. I have checked that two have different MAC address, and are assigned with different IP addresses. If the second box is turned off, the connection to the first box would come back.
At the moment, I don't know what else could be wrong.... Could you help? Thank you~

What connection do you maintain to the boxes? How do you know the connection to box 1 drops? How do you know it comes back?
In short, give us a few more specifics.

Let's assume you are talking about ssh, for example. If so, I would run the ssh server on box 1 with debug turned on, in the hope to see something. Or trace packets on box 1 (e.g., tcpdump port 22) to see what causes box 1 to sever the connection.
With ssh, though, I don't see how the connection can come back on its own. Once it's interrupted, it has to be re-established manually.

Sorry for being so vague yesterday. On both boxes, ESXi 5.1 were installed. During the test, I simply kept a continuous pinging to both hosts. As the second box started to respond to ping, the pinging to the first box began to show a few drops for first few seconds before becoming unreachable completely. Then, if the second box was turned off, the pinging to the first box would recover fully.
It seemed that the process of binding MAC to IP could be the problem, but I don't know what to do next. Thank you~

I don't know how to do this with ESXi. If it were Linux, I would trace ping packets on the receiving systems as well as on the ping originator and see what happens. The easiest tool to use is tcpdump.

Hello Burndbausch,
Maybe I found the cause of the situation... When I set up the first ESXi box, there were two NICs attached. The ESXi setup process teamed them together for failover. Subsequently, I removed one of the NIC to be used elsewhere. Here is the funny part: there is nowhere on the first ESXi showing it is still running in failover mode. So as soon as the other NIC showed up in the same network, it thought it should retire and consequently, disconnect itself from the network. Realizing this possibility, I reset the network configuration of the first ESXi box and that was it!
It was a very good lesson for me and thank you very much for the help