Uniquely identifying URLs with one 64-bit number

Question

This is basically a math problem, but very programing related: if I have 1 billion strings containing URLs, and I take the first 64 bits of the MD5 hash of each of them, what ki

Answer 1

You have tagged this as "birthday-paradox", I think you know the answer already.

P(Collision) = 1 - (2^64)!/((2^64)^n (1 - n)!)

where n is 1 billion in your case.

You will be a bit better using something other then MD5, because MD5 have pratical collusion problem.