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`