Hi anupjak,
Uninstall the Realtek LAN drivers from the system. Restart the unit and download the latest realtek drivers from the Dell site.
Driver Download page: http://dell.to/QjjZDA
(Key in the system service tag and the appropriate operating system do download the driver)
Also refer to the below thread for more information.
http://en.community.dell.com/support-forums/laptop/f/3518/p/19345850/19801153.aspx
