Connection to internet fails after restarting the modem

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http://askubuntu.com – Every time I restart my modem, I come across this problem: the modem is shown connected but no internet is available. ... This means that if I restart my modem, I should install a fresh copy of the Ubuntu in order to get access to internet :( My laptop has only Ubuntu 12.04 as its OS; the modem is a ADSL2+ DynaLink RTA1025WE and the browser is Mozilla Firefox. Is there anyone who knows how I can fix this? Thank you. (HowTos)