Servers typically have more than one network interface card. In fact, at my company most linux servers have 5 cards. Just the other day we had to troubleshoot network connectivity with a box that we could not get back up on the network. To make matters worse, we had no idea which physical interface was which. This is where 'ethtool' is your best friend. All we did was run the command below and we could see which physical card was giving us the problem.
ethtool -p eth2 5
The result of this command will blink the light on the network card eth2 for 5 seconds. Of course you can set whatever number of seconds you like. Next time you're not sure which card is which, fire up good 'ol ethtool.