Se você estivesse usando roteadores Linksys, poderia fazer isso com o roteamento estático ...
Static Routing
If there are multiple routers on your network, it is necessary to configure the unit's static routing functions. The static routing function determines your network's data paths. Use static routing to allow different IP domain users to access the Internet through your Cable/DSL Router. Click on the Show Routing Table button to view the current routing table.
To create a static route entry:
Select a Static Route Entry from the drop down list. The device supports up to 20 static route entries.
Enter the following data for the static route:
Destination LAN IP : Enter the network address of the remote LAN segment. For a standard Class C IP domain, the network address is the first 3 fields of the Destination LAN IP, while the last field should be 0.
Network Mask : Enter the Subnet Mask used on the destination LAN IP domain. For Class C IP domain, the Subnet Mask is 255.255.255.0.
Gateway IP : If the Linksys router is the main IP router to connect your network to the Internet, then your Gateway IP is the same IP Address as the Linksys router. If you have another router handling your network's Internet connection, enter the IP Address of your main Internet router here instead.
Click the Apply button to save your changes.
Isso deve permitir que computadores nas diferentes sub-redes vejam e se comuniquem entre si.