Hi,
There's quite a few different network shields available for the Uno. To answer your question best bet is to look at the available pins after the motor shield consumes the ones it needs, and see which of the network shields fits the bill. Since this is a rover do you really want it to be tied to a network cable though? If you meant wireless, there are options available for WiFi that only utilise power and the D0/D1 pins which are normally used for serial comms - it's unlikely your motor shield will need these.
Check shieldlist.org for a handy reference.
You'll find many of these shields also will have a choice of pins that can be used for different functions, so it's possible the default setup might not suit.
Cheers ! Geoff