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I know where to find 5-digit ZIP code boundaries. Do 9-digit ZIP code boundaries exist? If so, where might I find them?

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It's important to understand that ZIP+4 codes and even 5-digit ZIP codes do not represent geographic regions. ZIP codes represent groups of addresses or delivery routes. The +4 codes might narrow it down to a block, a building or single organization.

ZIP+4 data is available directly from the USPS (including a file that relates it TIGER) or from third parties who have taken the USPS data and repackaged presumably with value added. The USPS data does not include geometries. It will require processing on your part.

You're unlikely to find a free, up-to-date source of ZIP+4 data.

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Very good point about not making any assumptions about geographic region to ZIP+4, or 5-digit ZIP codes. E.g. in Manhattan, there are certain large buildings with many tenants that have their own 5-digit ZIP. A short distance away there may be a small cluster of one story buildings that share a ZIP+4 code. I'm not implying that the USPS makes assignments arbitrarily. Just agreeing that there are many criteria, rather than a zip code to geographic area correspondence. –  Feral Oink Aug 4 '11 at 7:23
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