The "Fast Lane" Answer
A ZIP Code is a group of five or nine numbers that are added to a postal address to assist the sorting of mail. A complete, 9-digit ZIP Code, including its additional four digits (called a ZIP+4 Code), is the designation of a delivery route. In other words, regular ZIP Codes indicate which USPS employees will be servicing the address, while the ZIP+4 will tell you which delivery route it will be on. (How to add ZIP+4 to an address.)
The "Scenic Route" Answer
Knowing what the last four digits of a ZIP Code are all about requires knowing what ZIP Codes themselves are all about. The Zone Improvement Plan was something that the USPS came up with to make it easier to ship letters and packages across the country. It helped "zone" the country according to how mail was distributed, which accelerated sorting and delivery.
As the US population has increased and scattered, it's been necessary to expand on the system, to make room for everybody and their dog. That's where the "+4" part comes in. We're getting ahead of ourselves, though, so let's start with the basics.
These are the codes you're familiar with. They look like this…
…and most commonly indicate a destination post office. Here's why.
If you're mailing a letter from Boston to Seattle, the postman in Massachusetts doesn't really care what the street address of the destination is. It's what you might call "outside his jurisdiction." He just needs to know which postman to send it to so that other postman can get it to that address.
A postal worker can only cover so much ground on a given day. And since the USPS has a standard of delivering in rain, sleet or snow, that rules out the possibility of doing the service of delivering in stages (some today, some tomorrow). That means that any given post office is only servicing what it can reach in a day. ZIP Codes reflect that.
Typically, a ZIP Code is tied to a post office; by that we mean, every one of the latter has one of the former. You might think of ZIP Codes as a mailing address for a particular post office. Some do handle more than one ZIP Code, but a one-on-one gameplan is the norm.
It's very important to note that ZIP Codes aren't "boundaries." They're a collection of delivery routes. They don't follow geographic or administrative boundary lines. This why some ZIP Codes cross city, county, or in some cases, even state lines. They follow where the delivery trucks go.
Some ZIP Codes are special cases. Among them are "military" ZIP Codes, which include everything from military bases (domestic or otherwise) to battleships at sea.
Then there are "unique" codes. Businesses and organizations sometimes get their own ZIP Codes, due to the volume of mail they send and or receive. These are frequently benefiting from bulk mailing discounts, since the organization usually has a mail department that (1) presorts mail before giving it to the USPS, and (2) distributes mail internally so the USPS doesn't have to.
In both of the above examples, these special ZIP Codes still follow the basic pattern. Whether the non-standard code is assigned because they fall outside of regular service areas, or because of the quantity of mail they traffic, the code still circumscribes an area of delivery like standard ones do.
ZIP Code +
ZIP Codes only get more accurate from there. Postal codes are often appended with a suffix. This suffix is a hyphen, followed by anywhere from 1-4 digits. Observe:
84604-1234 (we'll talk about this one next)
These suffixes indicate increased levels of specificity in where the letter is sent. The 5-digit code is basically city-level accuracy, while the ZIP+4 (which we'll talk about in a second) is within a block or less.
The reason we mention this in between step here is that not every place has a ZIP+4 Code. The vast majority of US addresses do, but some don't, meaning that sometimes the best accuracy you can get is a 6– or 7-digit code. Now of course, the postman knows how to bring it home from there, but if you're using your ZIP Codes for things like geocoding, it might throw off your interpolation, making the geocode less accurate.
Ah, here we are. 9-digit ZIP+4 Codes are the cream of the crop. They're only provided for an address once it's been validated and proven real. These codes indicate a specific delivery route, meaning the actual path the mail truck would travel in a single drop-off. Usually this is 10–20 homes or locations.
ZIP+4 Codes are also assigned to PO boxes; typically, the boxes each get their own, and it usually incorporates the box number. Unique ZIP Codes, like those assigned to military bases and ships, can have an impact on ZIP+4 Codes as well; in some cases, there's simply no +4 assigned.
Now, an important fact about ZIP+4: because it's based on delivery routes instead of more permanent boundaries, the +4 part of the codes change often. 5-digit ZIP Codes also change, but they do so infrequently; it's a lot less likely that you will be living in a ZIP Code when it changes. Not so for ZIP+4. The +4 on a ZIP Code can be changed as frequently as once a month, based on things like how many postal employees are working, or who is working what route, etc.
You're probably thinking, "Well then, how do I know what the code is, so that I can use it to claim all the fun benefits that come along with them (which we describe below)?" The answer is to employ the services of an address validation provider.
Validating an address requires that the address match the address in the database, and that means adding the ZIP+4 Code. So, once the address is validated and proven real, the USPS will provide the current ZIP+4 Code for the address. In other words, if you want to keep getting the discounts and other things that come with CASS use, you'll have to be consistent.
Benefits of ZIP+4
There are a number of reasons that ZIP+4 Codes are good omens for your shipping. For one, ZIP+4 Codes require validation; that means you know for sure an address is real if it has a ZIP+4 attached to it. For another thing, they can help get you those bulk mailing discounts.
A real important benefit to using ZIP+4, however, is delivery speed. Using ZIP+4 on your mail can speed up processing and delivery, sometimes by as much as two days. That's right, you're mail can show up faster if you label things right. Bet you're scrambling for those codes now, huh?
If you take away anything from this brief article, it should be three things: ZIP Codes indicate delivery routes, using ZIP+4 gets your mail there faster, and we at SmartyStreets can get you those codes. Our blindingly-fast address validation software provides the appropriate ZIP+4 Code for every address we process. Go ahead and try it out. Or you could call us instead if you'd rather talk to a real person. (We'd offer a fake person too, but we don't have one of those on staff).
Either way, we can help you ZIP your address, and ZIP it good.