The Chronicles of Narnia describes the world in which Narnia exists as one major landmass faced by "the Great Eastern Ocean". This ocean contains the islands explored in The Voyage of the Dawn Treader. On the main landmass Lewis places the countries of Narnia, Archenland, Calormen, and Telmar, along with a variety of other areas that are not described as countries. The author also provides glimpses of more fantastic locations that exist in and around the main world of Narnia, including an edge and an underworld.
Lewis, C. S., (1950) The Chronicles of Narnia, London: HarperCollins