The Postnet (Postal Numeric Encoding Technique) bar code was invented by the US Postal Office to encode ZIP information. Having your letter printing system Postnet-capable enables you to receive a discount on the postage. The U.S. delivery address coding can be of three forms (1) 5-digit ZIP; (2) 5-digit ZIP + 4 code (3) 11-digit delivery point code. So a valid Postnet bar code contains either 32 bars, 52 bars or 62 bars. Postnet is a numeric symbology. Different from most other symbologies, Postnet has requirements to the actual bar height and width. The information is encoded in the height and has nothing to do with the spaces.
You can set these parameters for Postnet: