USPS POSTNET
The USPS Postal Numeric Encoding Technique (POSTNET) symbology was developed by the United States Postal Service (USPS) to help sort business reply and courtesy reply mail and was also used for regular bulk mailings. It was typically used in conjunction with the USPS Postal Alpha Numeric Encoding Technique (USPS PLANET) symbology.
In 2009, POSTNET was superseded by the USPS Intelligent Mail barcode (IMb), and PLANET was superseded by IMb in 2013.
The symbol can represent a fivedigit ZIP Code, a ninedigit ZIP+4 Code, or an 11digit delivery point barcode (DPBC). The USPS developed DPBC to uniquely identify each of 115 million delivery points in the United States. Its use greatly reduced the time that it took carriers to sort letter mail prior to delivery.
POSTNET is defined in the USPS Designing Letter and Reply Mail document.
The symbol comprises the following elements:

Leading quiet zone

Start bar, also called a frame bar, which is a single tall bar

Encoded address information

Check digit

Stop bar, also called a frame bar, which is a single tall bar

Trailing quiet zone
POSTNET encodes data by varying the height of the bars in the barcode. All of the bars and spaces are the same width. Each character contains five bars, of which two are tall and three are short.
This symbology supports all numeric digits (09).
POSTNET contains a check digit, also called a correction character, that is based on the modulo 10 (mod 10) algorithm. This digit is added to the ZIP or DPBC Codes as follows:

For a fivedigit ZIP Code, the barcode is six digits long.

For a ninedigit ZIP+4 Code, the barcode is 10 digits long.

For an 11digit DPBC, the barcode is 12 digits long.
 If you do not provide exactly five, nine, or 11 digits in your ZIP Code or DPBC data, the data will adjust to the length of one of these numbers. This results in a barcode that contains different data than you intended. 
Unlike the other barcode symbologies, which are valid in a variety of different densities, the USPS has specified a very precise density range for the POSTNET symbology. The correct density is 21 bars per inch, plus or minus one bar. Each bar and space pair must be between 0.045 and 0.050 inches wide, for a density of about four characters per inch.
The correct heights for the bars are defined as follows:

The height of the tall bars is 0.125 inches, plus or minus 0.010 inch

The height of the short bars is 0.050 inches, plus or minus 0.010 inch
The correct widths of the bars and spaces are defined as follows:
POSTNET requires a quiet zone of 0.04 inches above and below the symbol and 0.125 inches to the left and right.
The scanners that are used by USPS to read postal barcodes are designed to look for the barcode in a particular area of the envelope. Therefore, if you do not position the barcode correctly, the scanner may have trouble finding and reading it.
The correct position of the barcode is specified as follows:

The left edge of the barcode must be between 3.5 and 4.25 inches from the right edge of the envelope.

The bottom edge of the barcode must be 0.25 inches (plus or minus 0.125 inch) from the bottom edge of the envelope.