[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!