A second NIC would allow the PC to do both. Never bridge (route) between them.
To allow the broadband (cable modem) to attach to the Hub and set the PC IP address [i.e 192.168.1.101] with subnet 255.255.255.0 for a range of addresses 2 to 254 (0,1,255 are reserved), would require the AS/400 to be at an address in the subnet to be visible to the PC. Since this a hub not switch the subnet problem can be sidestepped by making entries in ‘Hosts/LMHosts’.
If this is truly a hub and the IP address of the AS/400 hasn’t changed, there shouldn’t be any problem as long as you have your PC default gateway going out through the hub and you’re using the correct IP address. <b>IF</b> it’s truly a hub. IP traffic should simply flow through a hub.
If it’s not a hub, then please describe what it actually is.
I have a basic Linksys router for my house network and it uplinks to a DSL router through which my ISP assigns IP addresses. My AS/400s have fixed addresses [192.168.1.x]. This laptop has IP address 192.168.1.102. But my Linksys does NAT, so I have no idea what my external ISP-assigned address is. I have my router doing DHCP for the PCs inside.
Whatever address is handed out by the Linksys from its DHCP is handled by the Linksys via NAT whenever I need to communicate outbound.
My main PC does have two NICs — one goes to my Linksys, the other goes to a hub that sits between the Linksys and the DSL router. This allows me to have one connection that can be enabled/disabled as needed and that can connect (almost) directly to my ISP. It also can talk to the external side of my Linksys router through the hub. Technically, I can route out one NIC and in the other through the hub and my Linksys router as long as I know the NAT’d address — which will always be in the same subnet as my direct connection anyway. (My ISP will see to that of course.)
I’m not sure of the meaning of this comment above — <i>Never bridge (route) between them.</i> Seems kind of useless to have two subnets without bridging them. (If their addresses are in one of the <i>non-routable</i> ranges, bridging or NAT/routing are about the only conceptual methods of getting them to talk to each other.)