That's very loosely how IPv6 works. Your ISP will typically assign your router a prefix and will route any address starting with that 56 or 64 bit prefix to you. Then devices on your network pick the remaining bits and they get their full address.

Well IPv4 also used to work that way back before address exhaustion. What I'm describing isn't an arbitrary allocation of a subset of a single fixed bit width address but rather two (or more) entirely disjoint address spaces.