I've just upgraded one of my systems from 12.04 to 12.10. Now I can't connect on that system beyond my local network. Connections within the local network seem to work fine, and I can make nonlocal connections from other machines (like the one I'm asking this question from). I suspect that some routing information has been messed up, but I don't know where to look for it. It's not a nameserver problem -- pinging outside sites by their IP addresses doesn't work either.

I have another laptop next to this one, also running Kubuntu 12.10. On the one that can't connect, arp produces no output. On the other one, it produces

192.168.0.1 ether 00:23:69:fa:ce:ae C wlan0

On the working machine, the output of netstat starts with some tcp entries. On the nonworking one, those entries are absent.

I asked this question on the Ubuntu forum but haven't gotten any answers there. One further complication: since the troublesome machine has no outside connection, it's extremely difficult to download anything to it.

For what it's worth, "ping 8.8.8.8" produces "connect: Network is unreachable".

Update: after a lot of fiddling, I have my external world back. I don't know what the key action was, but the first indication of progress was that "ping 8.8.8.8" worked. At that point I still didn't have a working nameserver, so external URLs didn't work. But I did this (based on an online post, of course):

sudo dpkg-reconfigure resolvconf

and answered Yes to all prompts. That did the trick!!

Apparently my problem was unique, or close to it, since I couldn't find any online references to it: local net working, remote net not working, including explicit IP addresses. So I suppose that if no one else has this problem, no one cares about the solution!!