[ubuntu] Can ping to servers but cannot connect to them

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http://ubuntuforums.org – I use a DSL Connection with DHCP enabled. I have used windows 7 before but did not encounter any problem. Now, when I try to browse a specific site (e.g. forum.projanmo.com), I only get the waiting for.... message on the status bar. I have used Firefox, Opera, Chrome but none of them worked. Yet, I can ping them. What can I do? :( (HowTos)