Valid Visa Credit Card Numbers?

Credit card numbers are made up of a 16 digit number that is shown in 4 groups of four numbers. The first 6 numbers indicate which type of industry has issued the card, digits 7 through 15 are the account number of the cardholder, and the last digit is a digit that is called a checksum, or a number used to determine if the card number is valid. More information about how credit card numbers are made is available at