> a /56 subnet seems to be like 2001:0db8:0123:45::/56 but Actually, a /56 subnet must look like 2001:0db8:0123:4500::/56 i.e. the rightmost two digits of the fourth "quad" must be zero.  3 digits there are fine, provided it's something like ":400".

@jbessee74, my first thought when I read your question was "that's pretty shabby of your ISP to give you a /56 if you have 200 locations".  And the more I think about this, the more I'm convinced that I'm right.  And it precisely doesn't matter how s...

This gives us one block of hex digits, or 32-bits, to use for subnetting.This isn't quite correct; it is a block of hex digits, but that's "only" 16 bits.  So you cannot have as many subnets as there are addresses in IPv4.  But you can have as many (...