The http server does not default properly to index.html

If I try to reach the root index.html everything is fine: http://localhost:3000/ is properly displaying index.html but if I try to reach a sub folder with another index.html: http://localhost:3000/a/b/ , the server deliver the root index.html instead of the file a/b/index.html