can only connect to one network

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http://askubuntu.com – Just installed Ubuntu 12.04 a couple of weeks ago on my HP laptop (completely replaced Windows). Everything seemed fine until a few days ago when I was unable to connect to the internet with my wireless network at home. I also tried three other wireless networks and also tried to connect by ethernet cable. These are all networks that I have been able to connect to in the past with no problems. In each case it says that I am connected to the network, but when I open Firefox I get the message "Firefox can't find server at start.ubuntu.com". I also tried Google Chrome and that didn't work either. (HowTos)