I have a desktop HP ProLiant comp-server, with ubuntu 12.04 installed. It is connected via wire to my Comcast modem. A while ago, a video-card in HP stopped working, but after I pulled it out and then reinstalled, everything got back, with one strange exception: my wire connection to internet became very limited. I can easily get to most of Google and Yahoo sites, but to nothing else (an error msg "This webpage is not available"). When trying to use "ping" command to check online connection to my office comps, the feedback is "all packages lost"; similarly, all "ssh" connections were denied. Comcast guys, after checking my modem a few times, told me there nothing wrong with the modem -- the problem is mine. (BTW, I have two laptops, and their wi-fi works fine.) Any advice???
All right, I guess I am the most stupid guy around here. The issue was hardware related, or rather simply using a wrong jack. OK, there are two telephone-like wire jacks at the back of the comp, labeled simply 1 and 2. While reconnecting all the plugs & wires (after fixing my video-card problems), I plugged the wire connecting my comp to a modem into the jack #1, which by now turned out to be a mistake. After struggling with all kind of possible ( most of them imaginary) problems, a simple thought to switch the jacks came to my mind (i. e. using #2 instead of #1). It worked!!! ( It would be helpful if anybody explained to me what is a damn functional difference between those jacks...)