They merely respect guidelines of a valid credit card number.
I advise you not not to try to use these for any actual transactions, only for testing purposes!
Knowing the Major Industry Identifier (MII) and the Issuer Identification Number (IIN) we can prefix a credit card number and then randomly select the rest, as long as they comply to the MOD 10 algorithm.
For more complete testing data you can make up an expiration dates, a card holder name and possibly an address with a zip code.
The first digit of the issuer identification number is the major industry identifier (MII).
It identifies the industry where the card will be most used in.
You can also generate valid credit card numbers for specific Issuing Networks by utilising their particular prefixes.
If this digit is 9 the next three digits are the country code from ISO 3166-1.
The issuer identification number also known as the bank identification number (BIN) is the first six digits of the credit card number.
The MOD 10 algorithm is a checksum (detection of errors) formula which is the common name for the Luhn algorithm.
This formula has been in use to validate a lot of identification numbers besides credit cards since its development by scientist Hans Peter Luhn from IBM.