Linux guest network unreachable.

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Greetings. I'm setting up a new computer and trying to get an Ubuntu guest up and running. After booting up the guest, it is unable to connect to the Internet. When I try ping 8.8.8.8 in the terminal, it says that the network is unreachable, so this doesn't seem to be a DNS issue. I have tried both NAT and Bridged mode with similar results. The host has a reliable Internet connection.

I have another machine on the network which does not exhibit the same issues, so I believe this is not a routing issue. The other machine is also using NAT mode.

I have searched the forums and taken a look at the FAQ, but have not found any solutions. I have tried looking at the logs, but I don't really know what to look for, so I am at a bit of a loss there. I have been using VirtualBox for many years and usually networking is kind of plug-and-play for me, so I'm a bit stumped by this one. Please let me know what details I can provide and I appreciate any help in advance.

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A much delayed reply. I upgraded to the latest version of VirtualBox, which didn't seem to help.

So I thought it was worth copying a working virtual machine from my old computer to the new one. I tried using the instructions in the FAQ:
viewtopic.php?f=35&t=55003

I tried both Move and manually copying the virtual machine to my NAS, then to the new machine. I was able to open neither copy of the virtual machine on my new computer. It appeared that the Move technique did not actually make a copy of the disk. The manual copy did include the disk, but it did not open because seems to be because of it not using relative references to the files (the drive letters are not the same on both machines). But I'm not sure, that's just a guess.

I thought doing this would allow me to narrow down whether it was an issue with the setup of the virtual machine or something about the networking environment on the new computer.

[granada29]

Maybe if you post a log (zipped) showing the machine starting, a failed ping to 8.8.8.8 and then a clean shutdown of the VM someone may be able to help.

Are you doing NAT over the same type of interface (WiFi, ethernet) on both machines? It's hard to know what is going on from your description.

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@granada29 Yes, you're right. I should have posted a log file. I am attaching one to this message.

Both machines are connected to the same router using ethernet connection.