First 6 is shared across the cards a particular bank issues. Its not a secret as you can google the first 6 to find the bank or even the reverse google the bank to see what BINs they use. Last 4 is intended to be human visible so people can identify the card in use. When you factor in the check digit ruling out 90% of the remaining combinations, there are still 10,000 valid cc combinations plus an expiration date you would have to guess just to get a single acct.