At the risk of pedantry, it depends on whether their allocation is centrally coordinated. 4 billion IDs with central coordination isn't too bad, but you need a

At the risk of pedantry, it depends on whether their allocation is centrally coordinated. 4 billion IDs with central coordination isn't too bad, but you need a much, much bigger space to make collision-free, uncoordinated ID assignment practical. Corollary: in a room of 23 randomly selected individuals, there's a 1/2 chance that two individuals share a birthday. 365 days in a year, but only 23 samples gets you to 1/2 chance of collision. Look up the "birthday paradox" for more info on this surprising result.