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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
Zip 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
Everyone’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
IP 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 255.143.68.1. 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
Social 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.
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Comments (24)
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I don’t know if it has actually come to this yet in America, but you can have an anachronism within the SSN timelines, as SSNs are recycled after the citizen who held the number has been deceased for so long. However, my sister, born about 18 months before me, has a lower SSN than I.
posted by Joel on 6-22-2009 at 1:43 pm
The class organization of ip address space is no longer relevant. IP address space is now organized by addressing subnets. i.e 72.68.33.20/27
Thanks for the history lesson though.
posted by Hiro Protagonist on 6-22-2009 at 2:17 pm
Back in 2001 I participated in Who Wants To Be a Millionaire — Play It! at Disney Hollywood Studios.
Got all the way up to 125000 (points) and got stumped with the question what did ZIP stand for. If only Mental Floss had been there for me!
posted by Beth on 6-22-2009 at 2:38 pm
That’s not entirely accurate about Credit Cards:
The first digit or two determines what kind of card it is. Mastercard always starts with ‘54′, visa with ‘4′, amex with ‘38′ i believe. There are other cards out there that begin with ‘3′ (i think M/C now owns them).
As for the check digit, it is calculated based on the LUHN formula.
posted by mor on 6-22-2009 at 3:01 pm
Back in the day we did not get SSN’s when we were born. Boomers got them when they opened a bank acount, got their first jobs etc. So perhaps, for older americans, you might not be able to assume a birth order so much as an ‘applied for a card’ order.
I actually have a copy of my grandfather’s application for a SSN in 1936. Very cool
posted by JaneM on 6-22-2009 at 4:25 pm
Social security numbers beginning with 000 are temporary numbers. International students at my University get them when they first come to the US. There are rules governing how and when they get permanent numbers.
Do you know how to tell if your number is new or used? Look to see if there are small marks (-) on both sides of the number in the columns on the social security card. If there are no marks, you have an unrecycled number.
posted by Maggy on 6-22-2009 at 5:24 pm
Not true regarding the birthday for SSN — my younger sister (by 2.5 yrs) has a SSN that is 2 digits before mine — i am 6816 and she is 6814. How did that happen?
posted by Kristen on 6-22-2009 at 5:33 pm
The postal code system used in Canada has only one more digit than a ZIP code, but it is much more precise. The people who live across the street have different postal codes, so really all you need to mail a letter is the house number and the postal code.
We also dial 1 for long distance in Canada.
posted by Jordy on 6-22-2009 at 5:37 pm
The description of how and when the social security numbers are developed and used are most accurate in describing individuals born since the 1980s. Before this time many/most individuals did not apply for a social security number until they were preparing to enter the work environment. So many individuals did not apply for a social security card until they were in their mid to late teens. Additionally, when the first social security cards were issued and nearly every adult applied for and received a social security number withing a relatively short period, there are many exceptions to the explaination given here.
posted by Billy on 6-22-2009 at 5:38 pm
Not true about credit cards, “mor”, I have a MC that DOES NOT begin with 54.
posted by Ruth on 6-22-2009 at 5:45 pm
I love learning interesting facts about randoms stuff such as this.. we all tend to think there is a hiden code inside of every cap’n crunch box….
posted by Meg on 6-22-2009 at 6:03 pm
mor: thanks for your comment. Regarding the credit card digits, I have two separate MasterCard credit cards, one beginning with “51″ and the other with “54.”
posted by Casey Johnston on 6-22-2009 at 6:28 pm
With SSN’s no segment of a permanent number can be all zero’s. Also the prefix 666 is never used. (Yes, I had to write a validation routine for SSN’.)s
Originally when Area Codes were given out, the ease of dialing (with dial phone) was a major consideration. Zeros and high numbers took longer to dial, so the major cities got the area codes with really low combinations: New York 212, Chicago 213, LA 312, etc. States that only had one area code all had zero as the second digit.
posted by PartiallyDeflected on 6-22-2009 at 9:36 pm
While it is undoubtedly true that most people in the long ago days didn’t apply for a SSN until they were adults, given how little people tend to move around, it’s probably still pretty accurate for most people.
I have three children. Two were born in Virginia & one in Hawaii. However, they have one VA-prefixed SSN & two HI-prefixed ones. The middle child didn’t get her SSN until we applied for the youngest’s. There are a few reasons for this–for one, in spite of me requesting a SSN be ordered, the hospital didn’t do so. And then we spent several months arguing with the folks at the VA Social Security offices about what they would & would not accept as identification. This changed daily based upon whim, and often a document was no longer acceptable by the time we obtained it. Her birth certificate, for instance. The folks in Hawaii gladly accepted that (plus either her shot record or her baptismal certificate, I don’t recall which) and issued her a card with no trouble.
posted by Sabra on 6-22-2009 at 10:10 pm
I used the work at Amex and the 7th figit tells you what your pay period is. every 3 days of the month is assigned to a partivular digit 1-9. ie if your billing cycle stops between the 1st and the 3rd of the month you get a “1″ 4th-6th, you get a “2″. etc. I used to know all of them, but there are really only 4 numbers that are the sequential “account” number.
posted by Jay on 6-22-2009 at 10:51 pm
@ Kristen – your and your sister’s SSNs were probably applied for at the same time – I was born in 1970 and my brother in 1972 and we didn’t get SSNs until the early 80’s when we wanted to start playing the stock market with money we’d saved. Our numbers are within a few digits of each other.
I have twins who got consecutive SSNs…5621 and 5622…made it so easy to memorize them!
posted by Jenipher on 6-23-2009 at 1:41 am
For telephone numbers, you say 1 is the international dialing code for the US. That is not entirely correct. It should say the international dialing code for CANADA AND THE US.
posted by Wally on 6-23-2009 at 2:25 am
My amex doesnt start with 38 either Mor. Think maybe just the first digit remains the same for the card types.
posted by Lauren on 6-23-2009 at 4:32 am
My daughter was born in Germany to American parents and when we applied for the SSN we ended up getting one for the Alabama area. Kind of weird I think. I would have thought Washington DC or something like that. Im in the military…
posted by Lauren on 6-23-2009 at 4:36 am
OK – I think there is a mis-communication on the international dialing codes (to dial to other countries). To dial to the US or Canada from another countriy, you dial 001 (this would be for a direct call without going through a service). To dial to say Germany from the US, you would have to dial 01149 then the number with the leading zeros taken off (as German numbers all begin with 0). 1 is the code for the US and Canada, 49 is the code for Germany. 011 gets you to the international lines to dial out in the US, then you use the country code. I am not sure why you have to dial 011 from the US, but if you were dialing to the US from Germany, you can just dial 001….not sure why it is different. (I lived in Germany 14 years)
And I was born in Turkey to American military parents and my SSN is from Pennsylvania, where my father is from.
posted by Michelle on 6-23-2009 at 1:15 pm
halo halo halo halo………..
posted by zoran kralev on 6-24-2009 at 5:42 am
We applied for all four of our children’s SS cards when they their grandparents wanted to give them Savings Bonds. The oldest child and the two youngest received the same first five digits. The middle son received an entirely different set. I’m sure the child is mine and was never in the Witness Protection program.
posted by Patricia on 6-24-2009 at 9:45 am
Class A IP addresses are from 1-126, not 127. 127 is not used because the IP address 127.0.0.1 always is the computer you are using. Hence the welcome mats, shirts and other merchandise that say “There’s no place like 127.0.0.1″
posted by AmyD on 6-24-2009 at 1:20 pm
It’s only relatively recently that SSNs have started to be assigned at birth. Not too long ago, you had to apply for one. Thus, many people (myself included) have SSNs that do not reflect their place of birth but rather the place where they applied for an SSN.
127.0.0.1 — gotta love localhost. ;-) Great for when you need to put an arbitrary IP addy into some documentation, since it’s definitely bogus but legal. (Much like 555-1212, which shows up often in movies and books, but is actually directory assistance for a particular area code. Real, but harmless.)
posted by Calli Arcale on 6-30-2009 at 5:17 pm