Re: There's begillions
You can subnet a /64 but lots of things won't like it. Most won't care and if your up for static assignments for servers, it does make sense. The default assumption is that your device's hardware mac address fills up the lower 64 bits so you need the top 64 bits as a /64 network address.
I've tried to explain the IPv6 world as a /64 is much like a Class C /24 where everything on the network can talk directly to everything else. The /56 or /48 that your ISP may hand out is much like a Class B /16 where its split inside into Class C. The /32 that ISP get are more like the old Class A /8 where your have enough infrastructure that major parts are dual homed differently. At least a /56 can hit the global BGP tables so if your ISPs let you, you can broadcast parts of their blocks to your other ISP which is something very few would ever even consider in the days of IPv4/24.