23 January 2007
Last week I threw in some remark about the Luhn test, an algorithm used to determine the validity of most credit-card numbers, including all the ones you're likely to see. Today it's big news at Consumerist, so I figure it's time to fill in the gaps in the proferred knowledge.
First thing you need to know is that, for Visa/MasterCard anyway, the first six digits describe a specific issuer. (Major banks have more than one six-digit sequence associated with their operations. Many Bank of America Visa cards start with 4356xx. Mine, um, doesn't.) The string ends with a checksum; everything in between is an account number. This is, incidentally, how you can get a new card from the same bank and most of the digits are the same.
Incidentally, there was a time when Visa numbers in the 45xx and 49xx ranges seemed to be reserved for non-US accounts. This is no longer the case.
And MasterCard issues no 50xx cards, which perplexes some of our customers who heard somewhere that anything starting with a 5 is a MasterCard. (51xx through 55xx only, folks.) I have a Sears card that starts with 50xx; it's not usable as a MasterCard. Sears does have a MasterCard, but it's not in the 50xx sequence.
One last tidbit: Most bank cards have a three-digit verification code (CVV) on the back, printed on the signature strip. American Express uses a four-digit code on the front, except for gift cards.