Q: iMac late 2009 showing connected to Internet but no Internet

A couple days ago our Internet completely stop working. We called Comcast and they said that they had upgraded the service in our area and we needed to get a new modem. We got the modem with build in wifi on and it worked perfectly with our PC laptop and my iPhone 5. Then we connected my iMac through Ethernet and it said it was connected. I also tried to connect to the wifi and it said I was connected. Then when I tried to go on safari it said "www.google.com can not connect to the server because you are not connected to the Internet" I then went to network settings in system preferences and it said it was connected. I ran the Internet diagnostics and it said the Internet was working fine. I went back to safari and typed in the http://10.0.0.1 for the modem settings and I connected and typed in my password and could change any of the settings in the modem. I also said that the iMac was connected to the modem and was online. I then went and deleted the network preferences and restarted the computer and it still said that my imac was connect to the Internet but still no Internet on safari. I have contacted Comcast multiple time and they said that it was my iMac because all my other devices work fine with the wifi. My last resort is to do a clean install of mountain lion but I don't really want to have to back up my whole computer. I f you to help me out that would be great!

Thanks,

Gage