Eli5: How do they know if it is a valid card number?

Question

When you are asked to input your card information online, how is it that some websites can automatically know if you input an invalid number and others cannot, before payment is processed?Example: as soon as I typed the last number of my card wrong, it went red and would not allow me to continue before I corrected it.

In: 340

Answer 1

Because cards from different banks have set prefix numbers like every debit card at academy bank has the exact same first 8 numbers and all armed forces debit cards have the exact same first 8 digits. and different cards types start with different numbers and it’s set for all cards of that type, like all visas start with a 4 and all Mastercards start with 5. Card numbers can even tell you if it’s debit or credit or prepaid. They’re not random at all, there a system to it.