Next, I will take the credit card number (usually also a type of bank card) as an example to introduce its general structure and components. Please note that the following content is purely an introduction to the composition mechanism of the card number. It is not used for any illegal purpose, nor does it involve real card information. All examples are fictitious data and are for learning reference only. ──────────────────────────────── 【General structure of credit card number】 1. Major Industry Identifier (MII) The first digit usually represents the industry to which the card issuer belongs: • 1, 2: Aviation, finance or other special purposes • 3: Travel and entertainment (such as American Express credit cards usually start with 34 or 37) • 4: Visa card • 5: Mastercard • 6: Different card organizations such as Discover may have their own identification rules. 2. Bank Identification Number (BIN, also called IIN, Issuer Identification Number) The first 6 digits usually constitute the BIN: This part is used to identify the card issuer and help merchants determine which bank or financial institution the card belongs to during transactions. For example, a fictitious Visa credit card might start with "453987XXXXXX", where "453987" is the BIN. 3. The remaining digits of the personal account identification number (excluding the check digit) are assigned by the card issuer based on the customer's situation, and each bank has different allocation rules. These digits are usually used to identify different customer accounts internally. 4. The last digit is usually the check digit, and its calculation algorithm is usually the "Luhn algorithm", which is used to detect whether the number is entered incorrectly. This algorithm weights the previous digits and then calculates a value that makes the entire card number meet certain modular operation conditions. ──────────────────────────────── 【Example】 Suppose you construct a fictitious Visa card number (Visa cards usually have 16 digits): Example (fictitious, only for understanding the structure): • The first digit: 4 (indicating Visa) • The next 5 digits: 53982 (here as part of the identifier of the issuing bank. Generally, the BIN is the first 6 digits, so the first 4 is added to form "453982") • The next 9 digits: randomly assigned, used to indicate the customer account number, for example: 123456789 • Check digit: the last digit, calculated based on the first 15 digits and the Luhn algorithm, for example, it may be 6 The complete card number can be written as: 4 53982 123456789 6 Or written together: 4539821234567896 Note: – The numbers are for example only and do not represent the real card number. – The Luhn algorithm is a widely used verification algorithm. The basic principle is to weight each digit from the right. If the weight is greater than 9, the digits are split and added. If the final weighted sum is divisible by 10, the number is legal. – Different card brands (such as Mastercard, Discover, etc.) have their own number rules. For example, the BIN range of Mastercard includes 51~55 and 2221~2720, and the card number length is usually 16 digits. ──────────────────────────────── 【How to perform Luhn verification (simple explanation)】 1. Process the digits except the check digit from right to left. 2. Multiply the number by 2 for every other digit (i.e. the second, fourth, etc. from the right). 3. If the product is greater than 9, add up the digits of the product (for example, 8×2=16, 1+6=7). 4. Add all the processed digits to the digit that has not been multiplied by 2. 5. The check digit should make the sum divisible by 10. For example, suppose we have 15 digits, and the sum obtained through the above calculation is a certain value. In order to make the overall sum divisible by 10, the check digit is selected as the digit that can make the sum reach the integer difference of the next 10. ──────────────────────────────── 【Summary】 The composition of bank card numbers (such as credit card numbers) can be roughly divided into: – The first part: industry and issuing institution identification – The middle part: customer account number – The last digit: the check digit calculated based on the Luhn algorithm Through these compositions, we can understand that the card number is not just a string of random numbers, but contains a strict structure that is easy to identify the source and verify the integrity of the data. I hope this example can help you have a clearer understanding of the structure of bank card numbers. If you have more questions about a certain part of the details, welcome to continue the discussion!