THE ZIP CODE STRUCTURE AND HISTORY
In 1983, the U.S. Postal Service began using an expanded ZIP code system called "ZIP + 4", often called "plus-four codes". (Note 1)
A ZIP + 4 code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a city block,
a group of apartments, an individual high-volume receiver of mail or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery.
ZIP codes are numbered with the first digit representing a certain group of US states, the second and third digits together representing a region in that group
(or perhaps a large city) and the fourth and fifth digits representing a group of delivery addresses within that region. (Note 1)
Like area codes, ZIP codes are sometimes divided and changed, especially when a rural area becomes suburban. Typically, the new codes become effective
once announced, and a grace period (e.g., one year) is provided in which the new and old codes are used concurrently so that postal patrons in the affected
area can notify correspondents, order new stationery, etc. (Note 1)
Most significantly, in rapidly growing communities it is sometimes necessary to open a new sectional center facility, which must then be allocated its own
three-digit ZIP-code prefix or prefixes.( Note 1)
ZIP codes also change when postal boundaries are realigned.( Note 1)
HOW MANY ZIP CODES ARE AVAILABLE?
The zip code is composed of a 1 digit number, a 2 digit number, another 2 digit number and a 4 digit number. The total zip codes that can exist is based
on the number of 1 digit numbers, 2 digit numbers, 2 digit numbers and 4 digit numbers possible. The 10 digits used for a zip code are 0,1,2,3,4,5,6,7,8,9.
How many 1 digit numbers, 2 digit numbers, 2 digit numbers and 4 digit numbers are possible? We use math to find the answer.
Please refer to another article on this site for the details of how each group of numbers is determined for
a phone number; the mehtod is the same for the zip code.
Why Does My Telephone Area Code Have To Change!
Using the methods of math we can have 10 1 digit numbers, 10 X 10 or 100 possible 2 digit numbers, 10 X 10 or 100 possible 2 digit numbers
and 10 X 10 X 10 X 10 or 10,000 possible 4 digit numbers.
In summary, there are 10 possible 1 digit numbers each of which can have 100 possible 2 digit numbers,
each of which can have 100 possible 2 digit numbers and each of these can have 10,000 possible 4 digit numbers.
So 10 1 digit numbers X 100 2 digit numbers X 100 2 digit numbers X 10,000 4 digit numbers = 1,000,000,000 (1 billion) possible zip codes
Unlike the telephone number, each of which is assigned to a house or a person, and the social security number, each of which is assigned to a person,
the zip code which is assigned to a geographical area should have enough numbers available for the future.
The source for this article is:
Wikipedia - Zip Code