Three major airports serve the district. The closest is Ronald Reagan Washington National Airport, which is about 5 miles from the city and is primarily reserved for domestic flights, but is the least busy in the region. The busiest by international flights is Washington Dulles International Airport located about 24 miles away from the city center, and the busiest by total passenger boardings is Baltimore/Washington International Airport, about 30 miles from the city. Each of these three airports also serves as a hub for a major American airline: Reagan is a small hub for American Airlines, Dulles is a major hub for United Airlines and Star Alliance partners, and BWI is a major focus city for Southwest Airlines.