Since I spent the whole morning figurng out how to set this up properly, I thought spending the rest of the day documentating this would be a good idea. :-)
The premise is I am once again out in the wild. Lugging only my laptop and an external USB disk. Oh, and a GSM modem. Mobile broadband is now an essential working tool; no longer is it an excuse to claim that the restricted network of an office or organisation is crippling one's ability to access public Internet resources. Due to the added fact we have little reason to access the office network at the current location I am working in, the Ethernet and wireless adapters are disconnected.
Such a deal is fine if I work within the laptop system itself. But no, any contemporary software developer would have a variety of virtual machines running under Virtual PC or Virtual Server or VMWare. Yes, I know Hyper-V is out, but I only run Windows Vista on this laptop, along with Virtual Server 2005 R2 (SP1). Like any decent virtualization platform, Virtual Server allows hooking a guest machine's virtual network adapters to an actual physical network adapter present in the host OS. Which works great when you operate a static host that is plugged perpetually via its Ethernet ports or reliable wireless coverage. Not so great when it comes to my present situation - Virtual Server does not list my mobile connection as a viable network adapter. It's a dial-up after all. This presents a challenge in getting my guest machine to access the Internet. In asking around, Ken Schaefer once again provides clever suggestions.
The key connector is the installation of an MS Loopback Adapter. This device is visible to Virtual Server, allowing mapping a guest's virtual adapter to it. The next step is to enable Internet Connection Sharing (ICS) on the dial-up mobile connection, specifying the loopback adapter as the leech.
This setup automatically configures the loopback adapter to use the IP address192.168.0.1/24. That means virtual guests must follow suit and use addresses in the 192.168.0.0/24 range (subnet 255.255.255.0), defininig 192.168.0.1 as the default gateway. Make sure to tell Windows this new network identified in the MS Loopback Adapter is a private one. That is the fastest way to get the firewall to allow traffic between the host and the guests "embedded" within that loopback adapter. Or one can load the Windows Firewall control panel, select Change Settings and unchcck the loopback adapter in the Advanced tab. This lets traffic flow freely from guests to host. In my particular case, Windows Firewall is actually disabled because McAfee Security Center runs its own firewall and I had define unrestrictions in that software instead. The principle remains the same.
It is also good to jot down the IP addresses of the DNS servers of the ISP providing the mobile broadband subscription. The guest machines would not have any means to discover these on their own, so their virtual network adapter's TCP/IP settings should be manually configured with those IP addresses. Otherwise, navigating the Internet without DNS resolutions is plain suicide.
UPDATE: On further testing I found ICS provides a virtual DHCP service, providing other machines in the network with dynamic IP configurations. This includes default gateway and DNS server IP addresses. If you are not particularly into static IP address allocations, it is not necessary to define DNS - just switch to Obtain an IP address automatically for DHCP to assign it all.