The Postal Service developed the POSTNET (POSTal Numeric Encoding Technique) barcode to encode zip code information on letter mail for rapid and reliable sorting by barcode sorters (BCSs). The POSTNET barcode can represent a five-digit ZIP Code (32 bars), a nine-digit ZIP+4 code (52 bars), or an eleven-digit delivery point code (62 bars).
These are the nifty barcodes we see on the bottoms of envelopes and around the addresses on bulk mail. The theory is that these allow the postal service to deliver mail in 72 hours instead of 3 days.
Note : Postnet codes have a trailing checksum that is the Modulo 10 of the sum of the digits
