ZIP Code data is available through American FactFinder, NHGIS and the CensusCD products. However, ZIP Code boundaries are proprietary to the U.S. Postal Service and do not necessarily follow census geography. The Census Bureau uses ZIP Code Tabulating Areas (ZCTA's) as their equivalent to ZIP Code boundaries. Postal boundaries do not follow the same nesting as census boundaries. That is another reason the Census Bureau uses ZCTA's.