Air Canada is Canada's largest domestic and international airline serving more than 220 airports on six continents. Canada's flag carrier is among the 20 largest airlines in the world and in 2017 served more than 48 million customers. Air Canada provides scheduled passenger service directly to 64 airports in Canada, 59 in the United States and 98 in Europe, the Middle East, Africa, Asia, Australia, the Caribbean, Mexico, Central America and South America. Air Canada is a founding member of Star Alliance, the world's most comprehensive air transportation network serving 1,317 airports in 193 countries. Air Canada is the only international network carrier in North America to receive a Four-Star ranking according to independent U.K. research firm Skytrax, which also named Air Canada the 2018 Best Airline in North America.