The format and generation method of credit card numbers are defined in accordance with international standards (such as ISO/IEC 7812) and usually consist of the following parts： ### 1. **Structure of credit card number** -**Issuing bank Identification number (IIN)**: The first 6 digits identify the issuing institution (such as a bank or credit card company). -**Personal account identification**: The 7th to 15th digits (or less) are used to identify the cardholder's account. -**Check digit**: The last digit is used to verify the validity of the credit card number (through the Luhn algorithm). ### 2. **IIN range of common credit cards** -*****: Start with `4`, for example, `4XXX XXXX XXXX XXXX'. -**Mastercard**: Start with `51'to`55`, or `2221'to`2720`, for example, `51XX XXXX XXXX XXXX'. -**American Express**: Start with `34` or `37`, for example, `34XX XXXXXX XXXXX' or `37XX XXXXXX XXXXX'. -**Discover**: Start with `6011`,`65`, or `644'to`649`, for example, `6011 XXXX XXXX XXXX'. -**JCB**: Start with `35`, for example, `35XX XXXX XXXX XXXX'. ### 3. **Method of generating credit card number** -**Step 1**: Select the issuing bank identification number (IIN). -**Step 2**: Generate the personal account identification part in the middle to ensure that the total length meets the standard (usually 16 digits, Amex is 15 digits). -**Step 3**: Use the Luhn algorithm to calculate the parity bit. -**Step 4**: Combine all parts to form a complete credit card number. ### 4. **Luhn algorithm (check digit calculation)** The Luhn algorithm is used to verify the validity of the credit card number. The following are the calculation steps： -From right to left, multiply the even digits by 2. -If the product is greater than 9, the numbers are added (for example, `12` becomes `1 + 2 = 3`). -Add all numbers, including numbers that are not multiplied by 2. -Calculate the parity bit so that the sum is a multiple of 10. If the sum is already a multiple of 10, the check digit is `0`. ### 5. **Precautions** -**For testing only**: The generated credit card number can only be used for testing purposes and cannot be used for actual transactions. -**Legality**: The generation and use of false credit card numbers may violate laws or terms of service. If you need to generate a credit card number for testing, you can use some online test card number generation tools, or write a simple program to achieve it. Hope this information is helpful to you!