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

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

zipcode1Zip codes were invented by Robert Aurand Moon and by 1963 were widely used by the United States Postal Service. The five-digit number is a code for an exact location, with each successive digit indicating a more specific place. The first digit indicates a group of states; for example, a 1 directs mail to Delaware, New York, and Pennsylvania. The next two indicate a sectional center facility -- a zip code beginning with 108 directs mail to the facility New Rochelle, NY. The last two digits represent a village or town near the facility or a location within a metropolitan area. Typically in a non-metropolitan area a city gets the first area code, and surrounding villages and towns receive zip codes in alphabetical order (for example, Glenmont, NY has 12077 and Gloversville, NY has 12078). And in case you were wondering, ZIP is an acronym that stands for Zone Improvement Plan. 

3. Telephone Numbers

rotaryEveryone's a little more familiar with telephone numbers -- there's country code, necessary if dialing internationally (1 is the United States), and area codes, which indicate a broad geographic area. The next three digits indicate a smaller area, and the last four are a random permutation.  The area code and first three digits of a phone number are referred to in the telephone business as NPA-NXX.  These numbers convey a unit of purchase for telephone companies, as they will generally buy one NPA-NXX, or one combination. The ownership reveals why cell providers are often so tetchy about carrying a number from one to another, or vice versa: you would be stealing a phone number from one company and giving it to another.

4. IP Addresses

tcpip_ip_addressIP addresses, at their most basic level, identify individual computers to the Internet. They are a series of four numbers punctuated by periods that look something like Each of these numbers (such as 255 in the example) is referred to as an octet. Each octet can have a value between 0 and 255 (so if you see an IP address with any octet higher than 255, it's fake). Together the octets of an IP address contain information about the type of network and, to an extent, the location of a computer. The first octet, called the class, tells you the size of a network a computer is in. A Class A network has a first octet between 0 and 127 and can have over 16 million IP addresses; a Class B network has a first octet between 128 and 191 and have about 65,000 addresses; a Class C network, used for most homes, has a first octet of 192-223 and can have 254 addresses. There are also Class D and E networks with first octets of 224-255 that are used for more specialized purposes. Most IP trackers use a location database to determine where an IP address is coming from, so there is not a direct scheme for the other octets. However, due to the modern use of subnetworks within a network, IP addresses are often masked. Therefore, it is no longer directly possible to tell the type of network a computer hails from.

5. Social Security Numbers

sscardSocial Security numbers are nine-digit strings that most Americans are assigned at birth, and are generally used as an identifier as well as a qualifier for various kinds of insurance and income from the government.  The first three numbers tell where the person first applied for the card; if the card was applied for at birth and the mailing address used was also the residential address, the numbers tell the rough location of birth (doesn't apply to babies born during vacation in Panama, but in general this is true).  The next two digits are called the group number, and allow SSNs of the same area number to be broken into smaller groups.  They are assigned in the following order: odd numbers 01-09, evens 10-98, evens 02-08, odds 11-99.  The last four digits, the serial numbers, are assigned consecutively 0001-9999.

Big Questions
Why Does Turkey Make You Tired?

Why do people have such a hard time staying awake after Thanksgiving dinner? Most people blame tryptophan, but that's not really the main culprit. And what is tryptophan, anyway?

Tryptophan is an amino acid that the body uses in the processes of making vitamin B3 and serotonin, a neurotransmitter that helps regulate sleep. It can't be produced by our bodies, so we need to get it through our diet. From which foods, exactly? Turkey, of course, but also other meats, chocolate, bananas, mangoes, dairy products, eggs, chickpeas, peanuts, and a slew of other foods. Some of these foods, like cheddar cheese, have more tryptophan per gram than turkey. Tryptophan doesn't have much of an impact unless it's taken on an empty stomach and in an amount larger than what we're getting from our drumstick. So why does turkey get the rap as a one-way ticket to a nap?

The urge to snooze is more the fault of the average Thanksgiving meal and all the food and booze that go with it. Here are a few things that play into the nap factor:

Fats: That turkey skin is delicious, but fats take a lot of energy to digest, so the body redirects blood to the digestive system. Reduced blood flow in the rest of the body means reduced energy.

Alcohol: What Homer Simpson called the cause of—and solution to—all of life's problems is also a central nervous system depressant.

Overeating: Same deal as fats. It takes a lot of energy to digest a big feast (the average Thanksgiving meal contains 3000 calories and 229 grams of fat), so blood is sent to the digestive process system, leaving the brain a little tired.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at

More Details Emerge About 'Oumuamua, Earth's First-Recorded Interstellar Visitor

In October, scientists using the University of Hawaii's Pan-STARRS 1 telescope sighted something extraordinary: Earth's first confirmed interstellar visitor. Originally called A/2017 U1, the once-mysterious object has a new name—'Oumuamua, according to Scientific American—and researchers continue to learn more about its physical properties. Now, a team from the University of Hawaii's Institute of Astronomy has published a detailed report of what they know so far in Nature.

Fittingly, "'Oumuamua" is Hawaiian for "a messenger from afar arriving first." 'Oumuamua's astronomical designation is 1I/2017 U1. The "I" in 1I/2017 stands for "interstellar." Until now, objects similar to 'Oumuamua were always given "C" and "A" names, which stand for either comet or asteroid. New observations have researchers concluding that 'Oumuamua is unusual for more than its far-flung origins.

It's a cigar-shaped object 10 times longer than it is wide, stretching to a half-mile long. It's also reddish in color, and is similar in some ways to some asteroids in own solar system, the BBC reports. But it's much faster, zipping through our system, and has a totally different orbit from any of those objects.

After initial indecision about whether the object was a comet or an asteroid, the researchers now believe it's an asteroid. Long ago, it might have hurtled from an unknown star system into our own.

'Oumuamua may provide astronomers with new insights into how stars and planets form. The 750,000 asteroids we know of are leftovers from the formation of our solar system, trapped by the Sun's gravity. But what if, billions of years ago, other objects escaped? 'Oumuamua shows us that it's possible; perhaps there are bits and pieces from the early years of our solar system currently visiting other stars.

The researchers say it's surprising that 'Oumuamua is an asteroid instead of a comet, given that in the Oort Cloud—an icy bubble of debris thought to surround our solar system—comets are predicted to outnumber asteroids 200 to 1 and perhaps even as high as 10,000 to 1. If our own solar system is any indication, it's more likely that a comet would take off before an asteroid would.

So where did 'Oumuamua come from? That's still unknown. It's possible it could've been bumped into our realm by a close encounter with a planet—either a smaller, nearby one, or a larger, farther one. If that's the case, the planet remains to be discovered. They believe it's more likely that 'Oumuamua was ejected from a young stellar system, location unknown. And yet, they write, "the possibility that 'Oumuamua has been orbiting the galaxy for billions of years cannot be ruled out."

As for where it's headed, The Atlantic's Marina Koren notes, "It will pass the orbit of Jupiter next May, then Neptune in 2022, and Pluto in 2024. By 2025, it will coast beyond the outer edge of the Kuiper Belt, a field of icy and rocky objects."

Last week, University of Wisconsin–Madison astronomer Ralf Kotulla and scientists from UCLA and the National Optical Astronomy Observatory (NOAO) used the WIYN Telescope on Kitt Peak, Arizona, to take some of the first pictures of 'Oumuamua. You can check them out below.

Images of an interloper from beyond the solar system — an asteroid or a comet — were captured on Oct. 27 by the 3.5-meter WIYN Telescope on Kitt Peak, Ariz.
Images of 'Oumuamua—an asteroid or a comet—were captured on October 27.

U1 spotted whizzing through the Solar System in images taken with the WIYN telescope. The faint streaks are background stars. The green circles highlight the position of U1 in each image. In these images U1 is about 10 million times fainter than the faint
The green circles highlight the position of U1 in each image against faint streaks of background stars. In these images, U1 is about 10 million times fainter than the faintest visible stars.
R. Kotulla (University of Wisconsin) & WIYN/NOAO/AURA/NSF

Color image of U1, compiled from observations taken through filters centered at 4750A, 6250A, and 7500A.
Color image of U1.
R. Kotulla (University of Wisconsin) & WIYN/NOAO/AURA/NSF

Editor's note: This story has been updated.


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