Wifi hotspot through cell modem

I promised that I could set up a wifi hotspot for a meeting where there is no internet access. Pure hubris.

I ordered and received a cellular broadband modem in less than a week. It only talks to Windows, so I couldn't just mimic the configuration that I have with my home wifi configuration via Linux. I started by trying to figure out the right configuration for the hand-me-down Apple Airport that I would be using. That turns out to be a bad idea, since WinXP internet connection sharing demands that you have a pipe configured by your ISP at one end and the magic IP address of 192.168.0.1 at the other. There goes my preferred 10.0.0.0 network setup.

So here is the configuration that I found to work:

WinXP

cell modem: CHAP as per ISP-defined configuration

advanced network properties:

Allow other network users to connect through this device

Establish dial-up connection on demand

Shared with network: wired LAN

wired ethernet: 192.168.0.1 as per connection sharing rules. This is set automatically and cannot be changed.

Airport

ethernet uplink: DHCP

dhcp server off (it is provided by WinXP connection sharing)