Here’s everything you could ever want/need to know about zip codes! Who knew there was so much to be written?
http://en.wikipedia.org/wiki/ZIP_code

To further different the addresses. Somewhere around the world, there could be another 1234 Chestnut Lane 23456, but they would have a different ZIP code, a different last four numbers. Yours could be 2356, and theirs could be 7543. It’s so that they can expand the number of addresses they can use.

It simply breaks it down to a point where only a few houses share the same set of 9 numbers. The first three numbers determine the MSP (distribution for an area of the country). E.g. – 958 = Sacramento city, 956, 957, & 959 are “associate offices” and represent areas surrounding Sacramento. The next two numbers represent the office when the mail is delivered. E.g.- 95822 & 95831 = Sacramento, Land Park Branch. (My route is in the 95822 ZIP) The last four break it down further where, perhaps, the five or so deliveries that are located on the even side of the 4500 block of a street have the same ZIP+4 (as it is officially known). This code is sprayed on the mail at the main plant in the form of a barcode that breaks it down even more enabling machines to scan and place the letter in delivery sequence (DPS mail) and then into trays. This is then sent to the individual office where it is carried along with the manually cased mail that I put up and deliver every freakin day!

Sorry. Been carrying mail for the past 23 1/2 years. I’m just tired.

@chuck: awww, do you need a hug? :)

AstroChuck: We had a mail-delivery person conversation yesterday. In our previous neighborhood we LOVED our carrier. We all gave him baskets of gifts for holidays, we showered him with gifts when he got married and when he & his wife had their children. He was thoughtful, reliable & so friendly. In this neighborhood our carrier is a flat out b*tch. She never closes the mailbox doors so everything gets rained on and blows all over. There are three streets with the same house number – We all get each others mail every day. She stops and gossips with the older ladies in the neighborhood so we’re lucky if we see mail by 5pm. If it goes past 5pm, we get nothing…and she’ll swear that “you just had no mail today”. She refuses to come to the door for anything…you can be sitting in your house and you’ll get the missed deliver or signature required notice in your box. Most of us stopped giving her Christmas gifts when she was making comments like “I don’t want any more cheap crap” and “I’m gonna scream if I get any more freaking cookies”... Sorry to be off topic…