Why is the Ethertype field needed in the frame?

Question

Computer networks

They asked such a question at work, and all I could find was a couple of articles where it is written that this field indicates in which network layer protocol the frame itself will be encapsulated. But to the questions "why is this necessary at all" "give an example" and so on. I couldn't find answers. It would be cool if someone would tell on the fingers how it works.

Answer 1

This field is the network layer protocol index (ARP, IP, IGMP, ...).
ARP and IP protocol frames have a completely different format, so without this field you won't know how to handle it.
Up to the point that you can add your own protocol there yourself ... but which then you must process completely yourself. If you don't get bored, you can see how it's done here: Another virtual interface Edit .

Answer 2

https://en.wikipedia.org/wiki/EtherType - well, right in the wiki there is a table of possible values.