The first six digits of a card number (including the initial MII digit) are known as the issuer identification number (IIN). These identify the institution that issued the card to the card holder. The rest of the number is allocated by the issuer. Cards are issued by the issuer through an issuing network. The card number's length is its number of digits. Many card issuers print the first four digits of the IIN on their card, just beneath where the number is embossed, as an added security measure.
Although it is called a bank identification number, BINs are also used by other institutions, such as American Express. As increasing numbers of issuers are opting into the BIN network, the term issuer identification number (IIN) is increasingly used interchangeably with BIN.
The prefix system is also used to help identify potential security breaches or identity theft, by comparing such things as the location of the issuing institution and the address of the cardholder.