The road to Zettalinux

IPv6 was designed for 80 bits for network addressing (with 16 bits expected to be used for addressing within a site) and 48 bits for the autoconfiguration feature (basically, the ethernet MAC address, the longest station address of the era). More than a third of the network addressing was expected to be used for better network aggregation than IPv4; that is, to trade off the number of addressable networks for improved stability and small global routing table sizes.

In practice, much of that design had to be altered in practice.

Starting with IEEE pointing out their next-generation station addressing for their future ethernet follow-on protocol. This 'EUI' used 64 bits for station addressing. So barely was the design finalised before the intended addressing had to be altered to the (64, 64) split between network and station addressing which we have now.

I can't think of a single ISP which has used IPv6's in-service network prefix change feature, which was meant to allow even huge ISPs to advertise just one prefix. So plans for route aggregation haven't been as successful as hoped.

To reduce configuration error, routers moved away from being able to identify single bits in addresses (eg, moving from the 'subnet mask' notation to the 'prefix length' notation). This means that it's hard for sites to use their subnetting allocation to carry meaning (such as a particular bit-pattern meaning "no egress"). Whereas it's still possible to pick out selected bits in CPU's virtual addresses.