2002 Vauxhall Astra

A bit of my exhaust is hanging off. Not the end bit which is like a pipe sticking out of your car, but the part just before that, its like a metal box. The whole thing hasn't fallen, just what seams to be some sort of cover or guard which is just underneath it. It is dragging along the floor when I drive. The thing is very rusty so has obviously just worn away. Do I have to get a whole new exhaust? If not what is the name of the bit I have to buy? My car is a vauxhall astra convertable, not sure of the year, MV reg and has done 53000 miles