My preferred option on 2.0.0 and up is to create a 404 route and also allow a ** route path to resolve to the same component. This allows you to log and display more information about the invalid route rather than a plain redirect which can act to hide the error.
Simple 404 example:
{ path '/', component: HomeComponent },
// All your other routes should come first
{ path: '404', component: NotFoundComponent },
{ path: '**', component: NotFoundComponent }
To display the incorrect route information add in import to router within NotFoundComponent:
import { Router } from '@angular/router';
Add it to the constructior of NotFoundComponent:
constructor(public router: Router) { }
Then you're ready to reference it from your HTML template e.g.
The page <span style="font-style: italic">{{router.url}}</span> was not found.