Open the Terminal Window & type:
$ gksu network-admin
This should open the 'Network Settings' dialog. Click on the 'Connections' tab, select
'Ethernet Connection' . Click on 'Properties' Under 'Connection' make sure 'Enable this connection'
is checked. Under 'Configuration' leave it as 'DHCP'. Click "OK'. & again click 'OK'.
To check whether you are able to connect to the Net, from the Terminal Window, type:
$ ping
www.yahoo.com
This display some data like 64 bytes from
www.yahoo.com , IP address, time taken etc.,
Press 'Ctrl' + C to terminate ping.
If you see those data statistics , then your Net connection is working fine.
If your Net connection is not working, you'll get a message 'Unknown host www.yahoo.com'.