Networks

you can use a switch?

A simple switch will not keep these networks completely separated. A multiport router would be better, but would cost some bucks. Too bad you can't stuff another NIC in your equipment.

Like these on here:
http://www.buffalo-technology.com/products/wireless/wireless-n-nfiniti/

Unless the CE device has two separate ethernet (or WLAN) interfaces, this won't work.

First of all, if there are two separate LANs, you need an interface on the device to plug into each LAN. You cannot put a hub or switch to connect to both networks, or you've created a network loop, crashing the entire network.

When Multihoming a Windows machine, there are two big gotchas:

1) You can have only one active default gateway on the client device.
2) NetBIOS over TCP/IP (NetBT) binds to only one IP address per interface card.

Someone here suggested a simple router to connect to each network; that would not really work, since that would be very risky in terms of how that device is configured. Even with a router, which would allow you to multi-net off of a single ethernet port, the issue remains that you are connecting their two LANs together, and the issues with the default gateway and netbios binding are still there. I am certain that whoever manages their LAN would never approve of adding a router this way.

The best solution would be an A/B ethernet switchbox, or configuring the CE box with a second ethernet port. Note that even with a second port or a switchbox, , it would likely require manual intervention to switch the PC to the secondary LAN if the primary LAN went offline, as the gateway would be wrong and the netbios binding would need to be changed.