POSTNET (Postal Numeric Encoding Technique) 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 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 or frame bar and represented as the letter "S" in the USPS TrueType Font) and has a check digit after the ZIP or ZIP+4. 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 decimal, or "binary" 11000.)
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. 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, and it enables the Postal Service to sort mail into delivery point (address) sequence.
POSTNET is being deprecated, replaced by the Intelligent Mail barcode (also known as OneCode Solution) which combines all previous Postal Service barcodes and marking into a single barcode. Beginning May 2011, Intelligent Mail Barcodes will be required for mail to qualify for automation discounts.
The check digit is 10 minus (the digit-sum mod 10). That is, the check digit is calculated by:
If calculated correctly, the sum of the ZIP, ZIP+4, or ZIP+4+delivery point digits and the check digit, modulo 10, will always be zero. Continuing with the example above, (5+5+5+5+5+1+2+3+7+2) mod 10 = 0.
Note that the delivery point is often added after the ZIP+4 and before the check digit, in which case the computation of the check digit includes the ZIP+4 and the Delivery Point.
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