Mac Address. Use 'zeros' or 'O's

I am new to Ethernet and Arduino.

I have read through most of the basics and am trying to get the first sketch to work. https://www.arduino.cc/en/Reference/EthernetBegin

How do I know if I should use 'zeros' or 'O's for the Mac address?

on my board it says 90-A2-etc, etc

Should I use "90-..." or "9O-..." See the difference? It would be handy to have this distinction in the tutorials on this site. I know there are others who have mentioned this I just haven't found a straight forward answer.

Apparently you always use the numerical symbol, not the alphabetical symbol.

Not exactly written on the wall.

As the numbers are all hexadecimal, they can only be 0 to 9 and A to F.

It does not need to be "written on the wall".

Weedpharma

If all else fails, Google "Mac address": https://en.wikipedia.org/wiki/MAC_address

The standard (IEEE 802) format for printing MAC-48 addresses in human-friendly form is six groups of two hexadecimal digits, separated by hyphens (-) or colons (:), in transmission order (e.g. 01-23-45-67-89-ab or 01:23:45:67:89:ab ).

My emphasis.