# 5 Familiar Numbers and the Logic Behind Them

Given how digital the world has become, we are hardly bothered by having to deal with one string of numbers after the next: credit card numbers, social security numbers, IP addresses and so on. Do these numbers hold any meaning, or are they just random sequences in a database? Read on to find out.

## 1. Credit Card Numbers

TheÂ stringÂ ofÂ digitsÂ thatÂ makeÂ upÂ credit card numbers have a distinct, if subtle, structure. Â The first digit signifies which system it belongs to: 3 is for travel and entertainment cards like American Express, 4 is Visa, 5 is Mastercard, and 6 is Discover. Â The rest of the credit card number is used differently by each company -- for Visa cards, digits 2 through 6 are a bank number, 7-12 or 7-15 are the account number, and either 13 or 16 is a check digit, a number that is the result of a Â series of simple but generally secret computations with the other digits that helps verify the full number isn't fake. Â In an AmEx card, digits three and four indicate the type of card and currency, 5-11 are the account number, 12-14 are the card number within the account and 15 is a check digit (AmEx card numbers are 15 instead of 16 digits).