The lens fits because you have a half-format camera, you just have to calculate the figures x 1.6 = 16 - 28.80 mm. Since all lenses from the analogue era are always marked full frame :-) Greez KingRaven90

The 400D has an APS-C sensor (DX is Nikon's term for the same sensor size). The lens will definitely fit on your camera.
In general, you can mount any Canon lens on the 400D. If you put a full-frame lens on a camera with an APS-C sensor, the focal length will simply increase.