Because of a project I am consulting on a bit in my "free" time, I had to do some learning about credit cards. I won't go into the details on that project, but in my research I came across some interesting facts. These are taken from "50 Fun Facts About Credit Cards".
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- Credit card numbers conform to the Luhn algorithm, which is just a simple checksum test on the number. What you do is start from the right and double each second digit (1111 becomes 2121), then add them all together, and you should end with a number evenly divisible by ten. If it doesn’t, it’s not a valid credit card number.
- The first six digits will correspond to the issuer, including the major industry identifier. 34xxxx/37xxxx are for American Express, 4xxxxx is for Visa, 51-55xxxx is for MasterCard, and 6011xx is for Discover.
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