The United States Postal Service, which assigns zip codes, has issued about 45,000 of the 99,999 possible zip code values. Some zip codes, however, have no residents, such as a code assigned to a single large office building. The United States Census Bureau compiles statistics on nearly 32,000 regions it calls Zip Code Tabulation Areas, distinct areas approximating the boundary of a geographic postal zip code. With a total U.S. population of over 300 million, each tabulation area thus contains an average of roughly 10,000 people.
Charles, P. P., Shari L. P., Jonathan M. (2015). Security in Computing: Fifth Edition (pp. 615). Prentice Hall. Upper Saddle River, NJ