In aviation, a flight information region (FIR) is a specified region of airspace in which a flight information service and an alerting service (ALRS) are provided. It is the largest regular division of airspace in use in the world today. Some FIRs encompass the territorial airspace of several countries. Every portion of the atmosphere belongs to a specific FIR. Smaller countries' airspace is encompassed by a single FIR; larger countries' airspace is subdivided into a number of regional FIRs.