POSTNET (Postal Numeric Encoding Technique; the bar code format, not connected with the chain of shipping and printing stores called PostNet) is the format of bar codes that have been traditionally used on U.S. postal mailings, though it is in the process of being superseded by a new format, Intelligent Mail barcode.
Unlike other bar code systems which code based on bar thickness, POSTNET bars are distinguished by their length, either full or half. Digits are encoded by combinations of these bar lengths. Usually what is encoded is a zip code or ZIP+4 code, sometimes with two more suffixed digits to represent a delivery point. Full bars appear at either end of the code as a "guard rail" or "frame bar", and there is one checksum digit.
Bulk mailers apply bar codes to the pieces being mailed as one of the requirements of qualifying for discounted rates. The post office adds bar codes to other mail in the course of sorting.
In some cases, a PLANET code is used as well, to encode tracking information to supplement the routing information of the POSTNET code.
POSTNET codes have three varieties distinguished by the number of digits encoded, PostNet5, PostNet9, and PostNet11. These are used to encode 5-digit ZIP codes, 9-digit ZIP+4 codes, and ZIP+4 plus two more digits to represent a specific delivery point, respectively.