​Credit and debit card numbers follow specific formats to ensure proper identification and security. Typically, these numbers range from 13 to 19 digits. The first digit, known as the Major Industry Identifier (MII), indicates the card network and industry. The next six to eight digits form the Issuer Identification Number (IIN), identifying the issuing bank. The remaining digits, except the last one, are the individual account identifier. The final digit is a check digit, calculated using the Luhn algorithm to validate the card number. For example, Visa cards start with a 4 and have 16 digits, while American Express cards start with 34 or 37 and have 15 digits.

​The Luhn's algorithm, also known as the "modulus 10" or "mod 10" algorithm, is a simple checksum formula used to validate various identification numbers, such as credit card numbers. Developed by IBM scientist Hans Peter Luhn in 1954, the algorithm helps detect accidental errors in numbers. It works by doubling every second digit from the right, summing the digits of the products, and adding them to the undoubled digits. If the total modulo 10 is zero, the number is valid. While effective at catching common errors, it is not designed to protect against malicious attacks.