Cloning an Ubuntu guest on Virtualbox
Cloning a VM is easy in theory but if you don't set up networking properly in the newly cloned machine it can kill all networking on the host.
In particular, Virtualbox gives your cloned machine a new NIC with a new MAC code. This is probably what you want since you don't want multiple machines on your network with the same MAC address. But Ubuntu still has the NIC from the original machine in one of its configuration files, so there's a clash between the hardware created by Virtualbox and the Ubuntu VM's own view of the world.*
Simple but tedious to fix. Here goes, on the Virtualbox host machine:
VBoxManage clonevm original_vm_name --name new_vm_name --register