Source: U.S. Census Bureau, Geography Division. The Geography Division maintains a website of ANSI/FIPS codes at http://www.census.gov/geo/reference/ansi.html. Codes are available for states, counties, places, county subdivisions and other entity types.
American National Standards Institute codes (ANSI codes) are standardized numeric or alphabetic codes issued by the American National Standards Institute (ANSI) to ensure uniform identification of geographic entities through all federal government agencies. The American National Standards Institute (ANSI) has taken over the management of geographic codes from the National Institute of Standards and Technology (NIST). Under NIST, the codes adhered to the Federal Information Processing Standards (FIPS). ANSI now issues two types of codes (available from the link above). They continue to issue the commonly used FIPS codes, although the acronym has now changed to Federal Information Processing Series, because it is no longer considered the standard. The tables included on this webpage (link above) provide the FIPS codes, which are the codes most commonly used by the Census Bureau.
ANSI also issues the Geographic Names Information System (GNIS) Identifiers, which were established by the United States Geological Survey (USGS). The USGS provides a tool for searching the Geographic Names Information System for GNIS codes. It can be found on the linked page above under "Other Resources".