With 64 jets, Allegiant Air has achieved the lowest costs, fullest planes, and highest margins in the U.S. airline industry. It serves 75 small U.S. cities and faces competition on just 17 of its 203 routes. It offers a no-frills fare and charges for everything else, from carry-on luggage to water.

What type of monopoly is Allegiant Air? What would be the barrier to entry?