[brookst]

The key concept in routing is hierarchy, which Ethernet does not have. It’s an extra dimension that changes the way the protocol and applications work at that layer.

You can say a cube and a square are the same thing, but I don’t think that’s particularly useful pragmatically. Ditto for L2 and L3; if you abstract out the difference, they are indeed the same. But that’s not useful.

Now, you could say that time has brought evolutions / optimizations that blur layer boundaries, like VLANs at layer 2. But that doesn’t make the model less useful.

[fanf2]

In my experience working alongside network engineers, they use “layer 2” as a synonym for Ethernet, “layer 3” as a synonym for IP (v4 and v6), “layer 4” as a synonym for TCP and UDP, and “layer 7” as a synonym for application protocols. They don’t talk about the other layers. It’s just jargon, without any deeper meaning.

[brookst]

Well yes, those are the network layers, so network engineers think about them the most.

Admittedly layers 5 and 6 are kind of blurry, but so what? Doesn’t mean the model isn’t useful, just that it isn’t a perfect map=territory.