I've set up a virtual machine configured with bridge networking. After making eight clones of it, the new machines can't connect to the network.
I've instructed VirtualBox to re-generate the machines' mac-addresses.
In the Master machine, just delete them before you clone the new machines:
I encountered the same problem on a previous version of VirtualBox and I read somewhere that there was a bug which rendered the "generate new MAC address" tick box useless.
I'm now using Ubuntu 12.04 with VirtualBox 4.2.10. I noticed now that when I created the clone, there was no tick box to "generate new MAC address."
This gave me some problems even if I deleted 70-persistent-net.rules because the system would automatically generate the file on startup with the same MAC address as the original, which made it fail to connect.
But I discovered that in VirtualBox Manager, you can select the clone, go to Settings>Networking>Advanced and generate a new MAC address by ticking the box.
So what I did was to delete 70-persistent-net.rules first, shut down, generate new MAC address and start again, and now it works.
Alternatively, you can delete the file first before you clone. Then generate MAC address, then start the clone. That ought to work as well.