PostNET stands for Postal Numeric Encoding Technique and was previously used by the United States Postal Service (USPS) to sort, track, and identify mail. It is a two-state barcode, meaning bars can be in one of two states — tall or short. PostNET was replaced by the four-state Intelligent Mail barcode (IMb) in 2009.
PostNET codes were designed to represent a five-digit ZIP code, nine digit ZIP+4 code, or an 11 digit delivery point barcode (DPBC). These delivery point codes were created by the USPS to identify more than 115 million unique delivery points across the nation, and helped significantly reduce the amount of time carriers spent sorting mail prior to making deliveries.