B4R Question Ethernet-shield MAC address

[Mostez]

Hello,
I'd like to know if there is a kind of rule to compose MAC address for Ethernet shield, when I assign random MAC, sometimes I can not connect to network and I have to try again before I get it work!
As I know, any networking device should have MAC address by manufacturer, why should we assign MAC to this shield anyway and how can we get it's original MAC.

Thanks

 

[Erel]

There is no original MAC with the ethernet shield.

I don't think that there are any special rules related to the 6 bytes that make the MAC address.