Thread by @ErrataRob

What's the difference between a "routable" and "non-routable" protocol?

Correct answers only.

I mention this because googling the question gives handwaving by people who don't understand the answer.

Routers forward packets based on the address PREFIX (average around 20-bits of a 32-bit IPv4 address).

Ethernet bridges forward packets based on the entire 48-bit MAC address.

Thus, routing tables can handle 4-billion IPv4 devices with 1-million routing table entries.

As far as Ethernet switches/bridges are concerned, a MAC address is a random 47-bit number. Sure, it has a prefix assigned to the vendor, but it doesn't correspond to the location on the network, so is random as far as they are concerned.

Historically, routing protocols allow routers to remember complex topologies, allowing multiple paths towards a destination.

Historically, Ethernet switches use "spanning tree protocol" to temporarily disable redundant links to prevent loops from forming.

I say "historically" because Software Defined Networking kinda upends this, doing crazy things with Ethernet and IP headers that doesn't fit our historic model.

Ethernet is a "broadcast domain". Frames sent to FF-FF-FF-FF-FF-FF are received by every machine on the local Ethernet network.

There's no Internet-wide broadcast. Sure, [255.255.255.255] goes to every device -- but only on the local broadcast domain. Routers don't route it.

Ultimately (as shown with SDNs), the can be the same -- it's just that different tradeoffs are chosen, one optimized for billions of devices around the globe, and the other optimized for thousands of devices on the local campus (or building).

Probably the most important difference is IP is limited and Ethernet is universal. IP only works with the global Internet. Ethernet works with anything, it predates IPv4, and you can still use today's Ethernet switches in a future IPv7 protocol without upgrading them.

It's like asking the difference between the postal network and local roads. One is designed to be a global network with a wide range of transports, including boats and planes. The other is designed to be a local network, inside the local city. One is layered on the other.