Despite it's small size, this is the most difficult network design problem I've faced. There are three nodes in this network:

• PC running Windows XP with an internal WiFi adapter.
• Base station with both WiFi and a Wireless Modem (WiModem)
• Mobile device with both WiFi and WiModem

The modem is a low-bandwidth but high-reliability connection. We'd like to use WiFi for high-bandwidth stuff like file transfers when the mobile is nearby, and the modem for control information. Here's the tricky part: we'd like the wifi traffic to go directly from the mobile to the PC, as rebroadcasting packets on the same WiFi channel takes up double the bandwidth.

We can do that with a manual configuration by giving the both the PC and the base station two IP addresses for their WiFi interfaces: one on a subnet shared with the mobile, and one on their own subnet. The routes on the PC are set up so that any traffic going to the mobile via WiModem goes through the secondary IP address so that return traffic from the mobile also goes through the WiModem. Here's what that looks like:

• PC
• WiFi 1: 192.168.2.10/24
• WiFi 2: 192.168.3.10/24
• Default route: 192.168.2.1
• Base Station
• WiFi 1: 192.168.2.1/24
• WiFi 2: 192.168.3.1/24
• WiModem: 192.168.4.1/24
• Mobile
• WiFi: 192.168.3.20/24
• WiModem: 192.168.4.20/24

We'd like to move to having the base station automatically configure the mobile and PC, as the manual setup is problematic when you start having multiple mobiles and PCs. This means that the PC can only have 1 IP address and needs to be treated as being pretty simple. Is it possible to have a setup driven by DHCP on the base station that is efficient with bandwidth?