The first six for the card bin, used to identify the card issuing bank; in the back is the bank's own set of numbers; the last for the verification of the bit, is through all the previous number arithmetic;
Banks set up their own part of the card is generally issued by the branch number of the head, the branch number is the bank's own internal number, different banks are not the same.
The first six digits are: issuer identification code Issuer Identification Number (IIN).
The middle digit is: Individual Account Number Identification (from the seventh digit of the card number)
The middle digit is customized by the issuing bank and usually consists of 6-12 digits. A maximum of 12 digits can be used.
The rules for personal account number identification vary from bank to bank. Some banks will include code information such as branch, sub-branch, savings outlet, etc. in this section. Some banks may not be as detailed and will use the natural running water sequence code.
The last digit is the check digit
The digits in front of the card number are used to calculate the last digit of the credit or debit card using the Luhn algorithm.