[Dagger2]

Layer 3 exists as a layer of routing and aggregation on top of layer 2. Aggregation necessarily consumes address space, so L3 needs to be bigger than L2 to accommodate the full L2 address space. The L2 address space is 64 bits and the next power of 2 up from 64 is 128, so here we are.

96 bits would probably be enough too, but having large subnets has a few benefits -- it allows for securing NDP by using the extra space for a cryptographic key, and also it makes it much, much harder to scan for active hosts from outside the network.

Plus, can you imaging the wailing and teeth gnashing we'd be getting if v6 wasn't a power of 2 bits long?