本文发表在 rolia.net 枫下论坛A ZIP+4 code uses the basic five-digit code plus an additional four digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery. Use of the plus-four code is not required except for certain presorted mailings. In general, mail is read by a multiline optical character reader (MOCR) that instantly determines the correct ZIP+4 code from the address and — along with the even more specific delivery point — sprays a Postnet barcode on face of the mailpiece that corresponds to 11 digits. This technology has greatly increased the speed and accuracy of mail delivery and, in turn, kept costs nearly constant for over a decade.

For post-office boxes, the general (but not invariable) rule is that each box has its own ZIP+4 code. The add-on code is often either the last four digits of the box number or, if the box number consists of fewer than four digits, enough zeros added to the front of the box number to make it a four-digit number. However, there is no uniform rule, so the ZIP+4 code must be looked up individually for each box.

It is common to use add-on code 9998 for mail addressed to the postmaster (to which requests for pictorial cancellations are usually addressed), 9999 for general delivery and other high-numbered add-on codes for business reply mail. For a unique ZIP code (explained below), the add-on code is typically 0001.更多精彩文章及讨论，请光临枫下论坛 rolia.net