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Publication 28 - Postal Addressing Standards >
2 Postal Addressing Standards > 25 Highway Contract Route Addresses > 256 ZIP+4
When applying a ZIP+4 Code to a highway contract route address, an exact match is preferred. If a box number is included in the address, the mailpiece must bear the appropriate ZIP+4 Code representing the range for that box number. When box number information is not available, the highway contract base record must be used.