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.*

Postnet symbols encode 5, 9 or 11 digits only.

There is no human readable text displayed with this type of symbology.

Postnet encoding 5 digits
Postnet encoding 9 digits
Postnet encoding 11 digits

Text concatenation:
When a Postnet barcode element is generated by concatenating fields or counters, the following rules apply:
- Digits only are allowed.
- Only texts of 5, 9 or 11 digits will be considered.
When a value not complying with the above rules is found, the barcode is not displayed.

* From wikipedia.