Show how much you know! I've brought this problem up in the ubuntu forums and no help, full thread here: hxxp://ubuntuforums.org/showthread.php?t=670027

Basically, as quick as possible:
My ubuntu gutsy gibbon server install will not connect; no dhcp responses, no pinging the local router. All the ethernet interfaces show up just fine.
I've tried:
different cables
different ports
straight to the modem
through a bridge on a connected computer
a new nic
static and dynamic addressing
etc.

nic lights show proper connectivity.

At this point i'm thinking that my isp might be to blame. What do you think?

From your other thread, I don't quite understand your setup. Do you have a cable modem --> router --> linux box?

Your first output of ifconfig showed that you did have an address (71.199.49.91) though whether that was correct or not for the network, I don't know.

Sometimes you can't ping a router because the firewall tells it to drop or deny pings (not sure if that is the case or not in your situation) instead of allowing them.

You also need to make sure there are some nameservers in /etc/resolv.conf file. If you can ping -c 3 209.85.173.147 then you're on the net, but just need a nameserver since that IP address is for www.google.com.

Right now it's connected to a switch which is connected to the cable modem. Of course I can change that around as necessary. Actually right now i can't get my laptop running xp to connect, even though it was, so i'm trying to fix that first just to make sure another computer can connect to the modem.

I think that ip was a static address I assigned in that case, but not sure.

I will certainly check that file and ping when i get home. Do you have any ideas why the dhcp autoconfigure would fail? I know dhcp works on this router because my main windows box sets up just fine. Also, I validated the install disk so it should be fine.

I have read in some random place once that Comcast has blocked mac addresses that are new to the network, if i can't get my laptop to work i'm going to lean closer to that...

Thanks for your help!

You mention a router. Is it a combo router/cable modem? Because a switch does not assign IP addresses and as near as I can remember, you can't have multiple IP's from the same cable modem. So to serve more than one machine, you would need a router between the cable modem and the computers...

There's no router, i was just careless with my words. I suppose i'll put the linux box directly in the modem and pay better attention next time i take a networking class :P

You can have multiple IPs being served by one modem. A cable modem is nothing more than a layer 2 device, so it cares not about layer 3 information, such as IPs.

Sigh of relief! Problem has been solved after looking through the other thread going on right now about comcast. And to think i had read that solution before and decided the solution was too simple to solve my problems.

For future generations: Unplug your modem's power, plug in linux box cat5, plug modem power back in. This will apparently reset the mac address in the memory of the modem, and thus connectivity.