Check the man page for 'route' as the syntax varies between different versions of Unix. On Linux, for example, you would do something like:

route add -host <I>X.X.X.X</I> netmask <I>X.X.X.X</I> gw <I>X.X.X.X</I> dev <I>interface</I>


replace the X's as apparopriate with your target, netmask, and gateway values.