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Value	Encoding
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0	| | | | |

POSTNET is a barcode symbology that is used by the United States Postal Service to assist in directing mail. The ZIP Code or ZIP+4 code is encoded in this unique symbology that encodes data in half- and full-height bars. Most often, the delivery point is added, usually being the last two digits of the address or PO box number.

The barcode starts and ends with a full bar (often called a guard rail) and has a check digit after the ZIP or ZIP+4. The check digit is calculated as follows:

1. Add up all the digits being encoded, including the delivery point code if it is being used. If the encoded digit is a 0, add 0 (rather than 11, to which 0 is encoded). If you are sending a letter to somewhere in Young America, Minnesota, you might be sending to 55555-1234, which would have the sum of 35
2. Find the number that would need to be added to this number to make it evenly divisible by 10, in this case 5, which is your check digit.

The encoding table is shown on the right.

Each individual digit is represented by a set of five bars, two of which are full bars (i.e. two-out-of-five code). The full bars represent "on" bits in a pseudo-binary code in which the places represent, from left to right: 7, 4, 2, 1, 0. (Though in this scheme, zero is encoded as 11, or "binary" 11000.)

The above-mentioned example of 55555-1234 yields:

1. Sum is 35
2. The check digit is therefore 5
3. The data encoded will be 5555512345

Together with the initial and terminal guard bars, this would be represented as:

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Note that the delivery point is often added after the ZIP+4 and before the check digit.

There have been 4 formats of Postnet barcodes used by the Postal Service:

A 5 digit (plus check digit) barcode, containing the basic ZIP Code only, referred to as the "A" code. 32 bars total.

A 6 digit (plus check digit) barcode, containing the last 2 digits of the ZIP Code and the 4 digits of the ZIP+4 Code, referred to as a "B" code. Possibly 37 bars total. In the early stages of Postal automated mail processing the B code was used to "upgrade" mail that had been coded only with a 5-digit "A" code. This barcode was only found on mail that received a 5-digit barcode on the initial coding by an OCR. Now obsolete.

A 9 digit (plus check digit) barcode, containing the ZIP Code and ZIP+4 Code, referred to as the "C" code. 52 bars total. The 9-digit barcode enabled the sorting of mail to the individual delivery carrier, and in some cases into a semblance of delivery sequence.

An 11 digit (plus check digit) barcode, containing the ZIP Code, ZIP+4 Code, and the delivery point code. 62 bars total. This is usually referred to as the DPBC, or Delivery Point Bar Code. This is the predominant barcode in use currently (as of 2005), and it enables the Postal Service to sort mail into delivery point (address) sequence.

• PLANET

United States Postal Service, Domestic Mail Manual 708.4 - Special Standards, Technical Specifications, Barcoding Standards for Letters and Flats. http://pe.usps.com/cpim/ftp/manuals/dmm300/708.pdf