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I have the following visa card from comdirect.de:

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I guess:

  • Number A is my credit card number. The 4 groups don't have a meaning.
  • Number B: I have no idea. Might be my security code?
  • Number C is my security code.
  • Number D is eventually a card code (GD is Giesecke & Devrient)

So my question is mainly about number B and C.

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3 Answers 3

up vote 30 down vote accepted

Number A is my credit card number. The 4 groups don't have a meaning.

They do, actually. First digit is 4 for VISA, 5 for Mastercard, 6 for Discover/Diners Club, 3 for American Express/Diners Club (those are shorter than 16).

Also, first 6 digits for Visa and Mastercard are code numbers for the issuing institution. By these 6 digits anyone can know which institution issued the card, and what type of card it is (debit/credit, premiere or not, etc).

Number B: I have no idea. Might me my security code?

This is a security measure. These 4 digits must match the first 4 digits of your card number (the first 4 digits of the issuer code. The last 2 are card types for the issuer, though some different issuers may share the first 4 as well and only differ by the last 2). Amex cards don't have this (I'm not sure about Discover).

Number C is my security code.

Yes, this is called CVC or CVV2. It is used for card present transactions. The purpose of the code is to verify that a payment card is actually in the hands of the cardholder/merchant, for example when using the card over the internet or phone.

On American Express cards this is 4 digits, and they appear on the front of the card.

Number D is eventually a card code (GD is Giesecke & Devrient)

This is bank-specific, so I guess whatever you said...:-)

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On Discover, number B (the duplicated digits) appears on the back in the signature field. And it's the last four that are duplicated. In addition there's a number indicating which iteration of physical card this is, in case of reissuing a card (for loss or expiration of the old one). –  Ben Voigt Apr 7 at 17:00
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A little more information, the first 6 numbers are called the BIN number. The remaining 10 numbers are generated by an algorithm by the financial institution. A type of credit card fraud can result if a fraudster learns the algorithm allowing them to generate all the card numbers in a batch. To combat this most companies limit the number of cards generated with a certain algorithm. –  AxGryndr Apr 8 at 5:58

Number A is my credit card number. The 4 groups don't have a meaning.

There is a lot more structure to this number than you might think. There are digits (typically the first so many in the number) used for linking the card to the issuer for easier routing of payments, as each issuer has a globally unique issuer identification number (IIN).

A unique identifier for your card with the issuer will be embedded in there somewhere too, much like an account number; in fact, this could be your account number, or it could be a separate card-specific number which the issuer stores in a database to link your card to your account (think tracking multiple cards on a single account; also see the comments). See this other answer for further details on the make-up of the card number.

Another common property of the 16 digit number (for most cards) is use of the Luhn algorithm as a checksum, with the last digit computed from the other 15 as the check digit.

This helps provide an early warning in credit card number entry forms and other locations to differentiate between a card number which:

  • might be valid
  • definitely is not valid

A card number which fails the checksum can be automatically rejected without further ado as a mistype or a fake made-up sequence of random digits.

Of course, it isn't difficult with a little effort to invent a card number which successfully passes the check, meaning a successful checksum verification requires further confirmation of the card with the issuing authority to verify its authenticity and ensure sufficient funds are available.

Nevertheless, all the major card issuers use this algorithm for initial verification purposes of a supplied card number.

TL;DR: the long card number is not a sequence of random digits and certainly has structure to it.

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For Barclays, the last four digits on the card is the card issue number for the bank (not your personal issue number). When getting a new card, you can expect to have the same card number but however the last four digits will change (usually the last two, depends when you last got a new card). It can change from 5034 to 5042 for example. –  cybermonkey Nov 12 at 10:32
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@cybermonkey good points, I've updated the answer to make it clearer it isn't the account number per se, but could be a separate, unique identifier which links your card to your account in the issuer's database. (Having multiple cards on a single account is a prime example of where such a facility is needed). –  Cosmic Ossifrage Nov 12 at 18:54

The first digit tells you what type of business issued your card. The first six numbers identity your bank/issuer specifically. The seventh and following digits identify YOU. The final number checks you typed everything in correctly. You can also check whether any credit card number is valid using the Luhn algorithm. The Luhn algorithm is only in place to check for mistakes when typing your number in online or reading it out over the phone.

More here: http://redshed.co.uk/blog/how-do-credit-card-numbers-work/

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The first digit alone is not enough to tell the card scheme. Yes, every AmEx starts with a 3, but 3 can also correspond to JCB, Diners, or Maestro. 5 can be UnionPay, Interac, Mastercard, Maestro, etc. The Luhn10 check is not necessarily just for catching mistakes, but it's an easy way to determine if a number could be valid or not. If it doesn't pass, then there's no point in proceeding (could be a fraud attempt). –  Noah Oct 29 at 14:19

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