Mental Floss

5 Familiar Numbers and the Logic Behind Them

Casey Johnston

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

The string of digits that make up credit card numbers have a distinct, if subtle, structure.  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.  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  series of simple but generally secret computations with the other digits that helps verify the full number isn't fake.  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).

2. Zip Codes

zipcode1 /

3. Telephone Numbers

rotary /

4. IP Addresses

tcpip_ip_address /

5. Social Security Numbers

sscard /