The following are some public and common knowledge about the generation principle of bank card numbers (including credit cards, debit cards, etc.). All examples are fictitious data and are only used for popular science and education. They shall not be used for illegal purposes. ──────────────────────────────── 【1. Basic structure of bank card number】 According to ISO/IEC 7812 standard, most bank card numbers are composed of the following parts: ① Issuer Identification Number (IIN): usually the first 6 digits of the card number, used to identify the issuing bank or institution. ② Personal Account Identification Number: a string of numbers following IIN (the length varies depending on the bank and card type, and the total length is usually 16, 17 or 19 digits), representing the cardholder's account. ③ Check Digit (or Check Digit): usually the last digit, used to detect card number input errors, and its calculation method generally adopts the "Luhn Check Digit Algorithm". ──────────────────────────────── 【2. Introduction to Luhn Checksum Algorithm】 Luhn algorithm is a standard procedure for generating or verifying the checksum of the last digit of a card number. The process is as follows (taking a 16-digit card number as an example to explain the calculation of the last digit): Step 1: Starting from the right (excluding the checksum), multiply every other digit (that is, the second to last digit, the fourth to last digit, etc. in the card number) by 2. Step 2: If the product is greater than 9, add the digits of the product (equivalent to subtracting 9). Step 3: Sum all the digits (processed and unprocessed), and record the total as S. Step 4: The checksum is calculated as follows: if S mod 10 is not 0, then the checksum = 10 – (S mod 10); if S mod 10 is exactly 0, then the checksum is 0. During verification, sum the digits of the entire card number (including the checksum). If the result is divisible by 10, the card number is considered valid. ──────────────────────────────── 【3. Example description】 Here are two fictitious and common examples to illustrate the idea of generating card numbers: 【Example A: Generation of Visa cards】 1. Assume that the IIN assigned to Visa cards by a certain bank is 411111 (Visa cards usually start with 4). 2. The remaining part (excluding the check digit) is 15 digits (in this case, the card number is 16 digits). Set an account number: For example: 411111 111111 1111 X (X is the check digit to be calculated). 3. Calculate the check digit: ① Assume that the card number (excluding the check digit) is the first 15 digits of 41111111111111111 (for the sake of example, we use a common test number): 4111 1111 1111 111 ② Calculate the check digit according to the Luhn algorithm. The actual famous test card number: "4111 1111 1111 1111" is a verified valid number, so the check digit X is 1, and the combined card number is 4111 1111 1111 1111. (Note: 4111 1111 1111 1111 is a number commonly used for testing in many payment environments.) [Example B: Generation of MasterCard] 1. The IIN range of MasterCard usually starts with 51-55. For example, take IIN as 550000. 2. Set the account number. In this example, the total length is 16 digits. In addition to the IIN, a 9-digit account number and a 1-digit check digit are required: Assume the number is: 550000 000000 000X, where X is the check digit. 3. Calculate the check digit using the Luhn algorithm: Take the fictitious 5500 0000 0000 0004 as an example. The calculated check digit is 4, so the complete card number is: 5500 0000 0000 0004. (Again, this type of number is only for demonstration purposes and cannot be used for actual transactions.) ──────────────────────────────── 【4. Summary of the card number generation process】 1. Select a suitable IIN (specified by the card issuer, different card types and banks have fixed prefixes). 2. Assign the account number portion (this portion of the number is usually guaranteed to be unique and non-repeating by the bank's internal system). 3. Calculate the checksum based on the previous digits using the Luhn algorithm and append it to the end to get the complete card number. ──────────────────────────────── 【Important tips】 • The above algorithms and steps are public industry standards. Banks generate card numbers through a secure and regulated process in their internal systems. • These instructions are provided only for technical popularization purposes. Any use of this knowledge for fraudulent or illegal activities is illegal and may result in serious legal consequences. • The examples given here are fictitious data used to illustrate the generation logic, and are not numbers that can be used in actual transactions. I hope the above instructions can satisfy your curiosity about the rules for generating bank card numbers. If you have more questions, please feel free to discuss.