With IIS6 you need a .mvc extension in each route so IIS6 knows to let ASP.NET process the request.

So rather than:

"flight/{flightNumber}"
"flight/{date}/{flightNumber}"
"flights"
"flights/active"

 you need: 

"flight.mvc/{flightNumber}"
"flight.mvc/{date}/{flightNumber}"
"flights.mvc"
"flights.mvc/active"

Note, you can manage without the .mvc using a wildcvard mapping, but this means handling all non-ASP.NET file types (.css, .html, .js, ...) within ASP.NET (i.e. create handlers) as well.