Bank card numbers can be anywhere from twelve to nineteen digits long, while most credit cards have sixteen numbers. Is there meaning behind the codes? How do you know if a credit card is valid? How about the validation code?
The first digit of card numbers is the Major Industry Identifier (MII). It designates the category of the entity that issued the card.
0 – ISO/TC 68 and other future industry assignments
1 – Airlines
2 – Airlines and other future industry assignments
3 – Travel and entertainment and banking/financial
4 – Banking and financial
5 – Banking and financial
6 – Merchandising and banking/financial
7 – Petroleum and other future industry assignments
8 – Healthcare, telecommunications and other future industry assignments
9 – National assignment
The following 6 digits are the issuer identification number, which tells us which institution issued the card.
The 7th digit onwards represent a person’s account number, while the final digit is used for Luhn algorithm (checksum formula to validate a credit card). You can take a look at the inforgraphic below for detailed calculations.

Different cards have different security code names:
MasterCard – card validation code (“CVC2”)
Visa – card verification value (“CVV2”)
Discover – card identification number (“CID”)
American Express – “CID” or “unique card code”
Debit Card – card security code (“CSC”)

Graphic: www.mint.com