[immibis]

It's not about how many bits are 1 - it's about how many bits are important. And the first bits are always most important. So it's the first x bits.

If you have a /48 then 48 bits are used to determine the address is yours. Any address which matches in the first 48 bits is yours. If you have a /64, any address which matches in the first 64 bits is yours.

[patmorgan23]

It's about how many bits are 1, in the subnet mask.

[immibis]

The number of bits that are important is the number of 1 bits in the which bits are important mask, yes. I thought you couldn't remember how that mask worked.