What happened for the past decades is that every company used 192.168.0.0/24 or 10.0.0.0/16 for their internal networks, because that was the prescribed private

What happened for the past decades is that every company used 192.168.0.0/24 or 10.0.0.0/16 for their internal networks, because that was the prescribed private IPv4 space. But if everyone's using identical private address space, on physically separate networks, then you can't merge and route between those networks unless you NAT or renumber everything on them. The equivalent mechanism in IPv6 is Unique Local Addresses: https://en.wikipedia.org/wiki/Unique_local_address https://en.wikipedia.org/wiki/Unique_local_address and theoretically there should be no overlap or conflict with these addresses, that they can be safely merged together. In fact, some of the impetus has been removed to even use private network space in the first place, such as address space exhaustion, so if a company has been provisioned enough public IPv6 space for their whole network, they didn't even need to use NAT or ULA in the first place.