alt Hacker NewsThe 'why' behind all of these protocols and decisions is left out in favor of the cert exams. Being able to implement is usually what gets you paid.
>mere fact that Layer 2 and Layer 3 addresses are distinct concepts.
The logical separation between the ideas allows for a lot of flexibility, and there were some wrong answers on how scalable vs flexible things should be.
Ethernet and IP beat the shit out of everything else: https://en.wikipedia.org/wiki/Protocol_Wars
They emerged with a decent answer: L2 crosses intranetwork, L3 crosses internetwork. Their separation allows L3 addressing to be completely unaware of multiple changing L2 (lower level) intranetwork switches, which is immensely useful.
Trying to solve everything with One Master Protocol to solve it all turned out like this: https://xkcd.com/927/
this works nicely in IPV6, where true seperation of layer 2 and layer 3 exist. This does not exist in practice in ethernet, because ARP is required as a mechanism to learn MAC IP bindings and is dependant on IP AND Ethernet.
IPv6 solves this by doing neighbour discovery on link local addresses and multicast addresses.