The national personal identification number in the Netherlands is called the Burgerservicenummer (or abbreviated with BSN, introduced since november 2007). It is a 9-digit number where the number can be validated by a weighted 11-proof. Basically all the digits become a weighting factor and by calculating the sequential digits with their weight the final result must exactly be divisible by 11.
A nice effect of this weighted 11-proof is that there are at least 2 digits different between 2 individual numbers. You need to perform at least 2 changes to come from one number to another – it might be that there are 2 completely different digits (e.g., 112682765 and 112682777) or the you need to swap one digit and change another (e.g., 427096509 and 427096510).
Mathematically it might still be that there are two succeeding numbers like 427096169 and 427096170, which still need 2 changes to come from the one to the other. (more…)