If the network hardware or virtual environment (NIC) changes mac number Ubuntu will reserve the prevously used device name and assign the next higher device number to the "new" hardware.

For example, a machine with one NIC assigned eth0 changes the NIC hardware. On the next reboot, Ubuntu will reserve eth0 for the missing hardware and assign the new NIC as eth1.

To fix this and reset Ubuntu's memory so that the new hardware is assigned eth0, remove the file /etc/udev/rules.d/70-persistent-net.rules where the assignments are recorded.