the current 16-digit card number is an international standard, and the first 6 digits are BIN, which is allocated by international card organizations. Among them, the first one is VISA, the fifth is Master, Master,622126~622925 are UnionPay standard cards. 6 starts with China UnionPay single currency card (or American discover), 4 starts with international brand VISA credit card (the world's first credit card organization), 5 starts with international brand MASTERCARD, and 3 starts with Japanese JCB and AMERICAN EXPRESS.
the 7th-15th digits of each bank are allocated according to their own needs, and the 16th digit is the parity bit.
other non-16-digit card numbers are assigned by banks themselves.