ISPs are SUPPOSED to allocate a /64 for a single customer. Mine does, so I have 4.5 billion available addresses within my 2001:4653:nnnn:nnnn::/64 prefix...
Weren't they supposed to allocate a /48? Or did that change while I wasn't looking?
2^32 is ~4.3*10^9
2^64 = 2^32 * 2^32 , so quite a bit more