You already allways have a maximum number of IDs per core defined by the data type. I think putting the process identifying numerals at the front would be the smart thing to do because with that you can still count up IDs localy by adding 1 and processor wise by adding the offsett between processors. All you have to do is calculate this offset probanly using a macro like UINT_MAX was it in C I think which defines the Maximum value an uint can store and with that calculate the processor offset. Then the global cell ID would be processorID * offsett + localCellID