The serial number is divided into three parts (see Figure 1). The 8bit family code tells the Access System (and consequently the developer) what type of iButton is being used. The next 48 bits are lasered sequentially with no two numbers the same. The last 8 bits contain a Cyclic Redundancy Check (CRC) value that has been calculated across the family code and the 48bit serial number. The CRC ensures that iButton communication is error free.