Via
MakeUseOf.COM
You
may have heard before that credit card numbers follow a certain pattern
and structure so that they can be validated before a transaction is
accepted. However, it’s one thing to know that the structure is there
and another thing entirely to understand how credit card numbers work.
Why would this knowledge be useful? Well, if you run a small business
that doesn’t process credit card payments immediately, you could save
yourself money by ensuring the card details are valid. If you don’t,
well it could still be fun to show off your skills at parties. Here’s
how you do it.
Learning Where The Numbers Come From
Credit card numbers are not random. There’s a special set of numbers
to show information about the card issuer and another set to show
information about the card holder. One other number is also important,
but we’ll come to that later.
The very first number is the Major Industry Identifier (MII) and it tells you what sort of institution issued the card.
- 1 and 2 are issued by airlines.
- 3 is issued by travel and entertainment.
- 4 and 5 are issued by banking and financial institutions.
- 6 is issued by merchandising and banking.
- 7 is issued by petroleum companies.
- 8 is issued by telecommunications companies.
- 9 is issued by national assignment.
The first six digits are the Issuer Identification Number (IIN).
These can be used to look up where the card originated from. If you
have access to a list that details who owns each IIN, such as this
list of popular IINs on Wikipedia, you can see who issued the card just by reading the card number.
Here’s a few you might recognise:
- Visa: 4*****
- American Express (AMEX): 34**** or 37****
- Diner’s Club International: 36****
- Mastercard: 51**** to 55****
The seventh digit to the second-to-last digit is the customer account number.
Most companies use just 9 digits for the account numbers, but it’s
possible to use up to 12. This means that using the current algorithm
for credit cards, the world can issue about a trillion cards before
needing to change the system.
We often see 16-digit credit card numbers today, but it’s possible
for a card issuer to issue a card with up to 19 digits using the current
system. In the future, we may see longer numbers becoming more common.
The very last digit of each credit card is the check digit, or checksum. It is used to validate the credit card number using the Luhn algorithm, which we will now explain in detail.
The Luhn Algorithm Validation Check
The
Luhn Algorithm
is used to validate all sorts of numbers, including credit cards, IMEI
numbers and some social security numbers. It’s not designed to be a
cryptographically secure hash function, but merely a way to check errors
are not made when recording numbers. It is not foolproof, but is
generally considered to be useful.
Take the credit card number and read the digits from the right.
Double every other number and write them down – if you do it in the same
order as your card is written it will help with clarity. Now, wherever
you have calculated a double-digit number, change it so that it reads as
“first digit + second digit” (in other words, sum the digits of the
products). Finally, take your calculations and add those numbers to the
numbers remaining on your card that you didn’t double. A legitimate
credit card number will give you a result that is divisible by 10.
For instance, let’s use a number I’ve just made up: 4634 8932 1298
2767. I’ll enter it into a table to make it easier to understand the
steps.
Try it yourself using the card from the picture earlier in this article. What can you learn from it?
Original Article
http://www.makeuseof.com/tag/credit-card-numbers-random-read-understand/ 
