An intermediate-range ballistic missile (IRBM) is a ballistic missile with a range of 3,000–5,500 km, between a medium-range ballistic missile (MRBM) and an intercontinental ballistic missile (ICBM). Classifying ballistic missiles by range is done mostly for convenience. In principle there is very little difference between a low-performance ICBM and a high-performance IRBM, because decreasing payload mass can increase the range over the ICBM threshold. The range definition used here is used within the U.S. Missile Defense Agency.