Some of the latest Laptops especially Dell Inspiron 5420 and Lenovo G580 series comes with Atheros Network driver which has some problem with Ubuntu 12.04. The problem is that it will not detect the wired network connection(eth0) even though wireless connection is working fine. We can solve this issue by installing the proper network driver.
First you have to download the latest driver from here http://linuxwireless.org/download/compat-wireless-2.6/
Open Terminal (Alt+Ctrl+T) and extract the downloaded file using following command.
tar -xvf compat-wireless-2012-07-03-p.tar.bz2
Copy and paste the following commands one by one to change the working directory and install the driver
cd compat-wireless-2012-07-03-p scripts/driver-select alx make sudo make install sudo modprobe alx
Done. Now you should be able to connect to the Internet using wired network.