The IATA, the FAA, and various booking engines have assigned "city codes" which represent several airports serving a metropolitan area. Not every region served with multiple airports is assigned a location identifier, however, nor area all IATA-recognized facilities necessarily included; furthermore, the inclusion or exclusion of a facility may depend on the booking system.
Metropolitan area airport codes are handy for simplifying flight searches into or out of a region. Some booking engines which include airports based on distance from a point can be tricked into extending the search through use of the city code.
Extra caution is advised when booking connecting flights, however, as the inbound leg may arrive at a different airport from the outbound leg, necessitating a ground transfer that may be costly or time-consuming. In particular, many major cities channel most international flights through one airport and most domestic or short-haul flights into a different one.
Smaller domestic airports are more likely to be served by low cost airlines but that might also force you to re-check your luggage. Ask your airline if they allow interlining tickets — checking your luggage through to the final destination on multiple tickets.
List of airport area codes
This is not a listing of all airport codes; this is a list of special codes that cover multiple airports.
Airports are listed by decreasing importance for international travel. Some flight search engines might include not all airports into their queries.