When you add a new NIC to the VLM and reboot the machine, addresses are assigned to the NICs based on the order they are detected during the device scan. There is no guarantee that the NICs will retain the addresses they had previous to the reboot.

Thus, if you had a VLM setup with NIC0 assigned to eth0 add NIC1 assigned to eth1, after adding a third NIC, NIC3, and rebooting, there is no guarantee that that NIC0 and NIC1 will remain assigned to the interfaces they were assigned to previous to addition of an extra NIC. For example NIC0 could end up assigned to eth1 and NIC 1 assigned to eth2.

This will not occur if no additional NICs are added. For further information on ensuring that the ordering remains constant, please refer to the Oracle VirtualBox documentation.