2,6,A or E is what the second digit of your MAC address must be otherwise a lot of OS's will not work. They will say the NIC is down/not connected even though it is.
This is because if the second digit is not set properly it will not view as a valid MAC or device uplink. This is especially an issue with VMs whether in QEMU etc.. if you are making your own MAC.
MAC address is invalid
To fix it just........