Imac won't connect to internet with ethernet cord

Hello!
I need some help with my Imac if anyone can give me some advice. I have owned this particular Imac for over three years and this is the first time I have ever encountered this problem...
My Imac won't connect into my internet with my ethernet cord... It works perfectly fine and can connect to the internet wirelessly (I know this because a few days or so it connected to my house's wireless internet just fine) and when I moved back to my college it seems as if it won't connect to the internet through the ethernet cord.
I have the airport off and the internet as well as the ethernet cord works fine because I can connect to the internet on my netbook through the ethernet cord. My Imac also reads the ethernet cord because when I go into network preferences it registers that my ethernet cord is connected....
I am a little stumped on this particular problem and would love some assistance, my dorms do not provide wireless internet and my Imac is my main device to do all of my school work.
Thanks!

I'd have a word with your admin at college.

At the colleges my kids attend they can only have one device registered and connected by ethernet. If yours is the same and your netbook has been registered it may explain why your iMac won't connect.

Another thing to check is that the ethernet settings in the Network System Preferences panel is set to use a DCHP assigned IP address and not a static IP.