Adding routes manually to the routing table or forwarding table

You can always add routes into the routing table manually on every router even your home wifi routers by WEB interface or command lines.

By controlling the router's forwarding table manually, you will understand the routing process more.  By default, routers will put all subnets connected to its interface into the routing table. Thus you do NOT need to worry much about those connected routes.

You should only think about how the packet gets to the destination. The importance thing is this process is only ONE WAY. You should also think about how the packet gets back to its source.