Credit Card Numbers Are Not Random: How To Read & Understand Them Yourself

You may have heard before that credit card numbers follow a certain pattern and structure so that they can be validated before a transaction is accepted. However, it's one thing to know that the structure is there and another thing entirely to understand how credit card numbers work.

Why would this knowledge be useful? Well, if you run a small business that doesn't process credit card payments immediately, you could save yourself money by ensuring the card details are valid. If you don't, well it could still be fun to show off your skills at parties. Here's how you do it.

Learning Where The Numbers Come From

Credit card numbers are not random. There's a special set of numbers to show information about the card issuer and another set to show information about the card holder. One other number is also important, but we'll come to that later.

The very first number is the Major Industry Identifier (MII) and it tells you what sort of institution issued the card.

• 1 and 2 are issued by airlines.
• 3 is issued by travel and entertainment.
• 4 and 5 are issued by banking and financial institutions.
• 6 is issued by merchandising and banking.
• 7 is issued by petroleum companies.
• 8 is issued by telecommunications companies.
• 9 is issued by national assignment.

The first six digits are the Issuer Identification Number (IIN). These can be used to look up where the card originated from. If you have access to a list that details who owns each IIN, such as this list of popular IINs on Wikipedia, you can see who issued the card just by reading the card number.

Here's a few you might recognise:

• Visa: 4*****
• American Express (AMEX): 34**** or 37****
• Diner's Club International: 36****
• Mastercard: 51**** to 55****

The seventh digit to the second-to-last digit is the customer account number. Most companies use just 9 digits for the account numbers, but it's possible to use up to 12. This means that using the current algorithm for credit cards, the world can issue about a trillion cards before needing to change the system.

We often see 16-digit credit card numbers today, but it's possible for a card issuer to issue a card with up to 19 digits using the current system. In the future, we may see longer numbers becoming more common.

The very last digit of each credit card is the check digit, or checksum. It is used to validate the credit card number using the Luhn algorithm, which we will now explain in detail.

The Luhn Algorithm Validation Check

The Luhn Algorithm is used to validate all sorts of numbers, including credit cards, IMEI numbers and some social security numbers. It's not designed to be a cryptographically secure hash function, but merely a way to check errors are not made when recording numbers. It is not foolproof, but is generally considered to be useful.

Take the credit card number and read the digits from the right. Double every other number and write them down - if you do it in the same order as your card is written it will help with clarity. Now, wherever you have calculated a double-digit number, change it so that it reads as "first digit + second digit" (in other words, sum the digits of the products). Finally, take your calculations and add those numbers to the numbers remaining on your card that you didn't double. A legitimate credit card number will give you a result that is divisible by 10.

For instance, let's use a number I've just made up: 4634 8932 1298 2767. I'll enter it into a table to make it easier to understand the steps.

Try it yourself using the card from the picture earlier in this article. What can you learn from it?

How To Hide Your Personal Information On Facebook

In the age of the Internet, privacy is a luxury and you have to constantly be on your heels to maintain it. Facebook makes it particularly hard for users to guard their personal data. Despite many changes, Facebook privacy settings remain complicated. Moreover, past updates have unexpectedly made previously private information available to the public.

Well, no stranger needs to know your email address, phone number, or see your family pictures. In this article I show how you can check what the public can see of you on Facebook. I also explain how you can control which of your information is visible.

What Information Can The Public See?

Do you think your profile is all locked up and the public cannot see anything they shouldn't? Well, how about we check it out, just to be sure!

Go to your Facebook Timeline, i.e. the page that is shown when you click your own name on Facebook. While you are logged in, you will of course see everything. In the top right of your profile summary that heads the Timeline, you should see a Settings icon next to the Activity Log button. Click Settings and select View As...

This reveals what your Facebook Timeline looks like to a complete stranger. You can also test the view for specific people, by entering their names in the respective field in the top left.

The Timeline is one thing, but what you should really check is your About page. So while viewing your Timeline from the public's perspective, click About in the bottom left of the header. This is where Facebook potentially reveals your contact information, your employer, your education, where you live etc.

Something else you should check is what other parts of your Facebook profile are available to strangers by clicking the About button in the top left. This could include your friends list, your photos, your likes, subscriptions, events you are attending, your notes, and music.

So did you spot anything you didn't want the public to see?

How Can I Remove Items From Public View?

For most content, Facebook makes privacy control fairly easy.

Let's start with your About page. Return to this page in your normal logged in view. Notice there is an Edit button in the top right of every box. An exception is your history because it only aggregates available information. Click Edit for an item you want to change, then click the little symbols next to each entry to control who can see this information. You can choose from a few default options, including Friends or Only Me . You can also make the information available to custom lists of friends. Don't forget to hit Save every time you change a setting.

Curious about how to create custom lists of friends? Find out how you can use Facebook Friends Lists For Interests Or Circles.

Next, we will look at Photos. You can control the privacy for each photo individually. This is good to know because you cannot control the privacy level for the Profile Pictures album. To change its visibility, open a photo and click the Editbutton on the right. Now you can change the description, location, date, and select who can see the photo. Click theDone Editing button to save your changes.

Fortunately, you can easily change the visibility of your custom photo albums. You will find the menu icon to change who can see an album, next to the number of photos in the respective album.

If you do not want others to see photos you were tagged in, review the 3 Things You Need To Know About Photo Tagging In Facebook to find out how to control tags.

Finally, to control what else people can see of you, including photos you were tagged in, let's look at your overall Privacy Settings. In the top right of your profile, click the little arrow and select Privacy Settings. This page allows you to customize privacy settings ranging from who can find and contact you on Facebook (How You Connect) and who can see posts you have been tagged in to social ads.

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