A postal routing decision
ZIP Codes support the sorting and delivery of mail. Their assignment reflects postal operations and delivery arrangements rather than a universal grid of equal-sized geographic areas.
Why changes happen
Population growth, new delivery points, route changes, facility adjustments and operational needs can lead to changes in postal addressing information. Some codes may be introduced, modified or retired over time.
Why datasets may differ
Public directories and commercial datasets can have different update schedules, field definitions and coverage. A code may appear active in one source while another source has not yet incorporated a recent change.
How to verify current information
For a specific address, use USPS ZIP Code Lookup or another authoritative postal source. For research, record the dataset provider and retrieval date so readers understand when the information was collected.