[gentoo-user] strange network problem

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Hi ,everybody 
         I've met a strang network problem.My gentoo Netbook can't access google and some other web sites after  lying idle about more than half an hour's. But it can acesses  other sites normally ,And can pinging  ervery sites including google very well! The Only thing i can do is rebooting  the system,and Network resume aftre that. I've googled a lot,and found nothing to solve this problem. :-( 
       It borthered me a lot.Please help me!