How to share Windows 7 Internet connection AND Extreme USB hard disk?

Question

Hi All,
I'm trying to share the mobile broadband Internet connection on a Windows 7 PC as well as the USB hard disk connected to the Airport Extreme (2nd Gen) with other WiFi devices. The problem is that I can get either the one to work or the other, but not both.
When I configure the Airport Extreme for DHCP+NAT, all machines on the Airport network can see each other, share files, etc. When I enable Internet Connection Sharing on the Windows 7 machine, the only way I've gotten that to work is to set the Airport Extreme to bridge mode, but then file sharing of the Airport Extreme's disk no longer works.
Device info: Internet-connected Dell Latitude E5420 Windows 7 Professional 32-bit, Airport Extreme 802.11n (2nd Gen), Windows XP Home Edition + Nokia N8 + etc. WiFi devices.
I've read all the manuals I could find and searched these forums but haven't been able to figure out a solution. I'm really getting desperate and will really appreciate any and all suggestions!
Many thanks,
Francois

Answer 1

I've found a solution that works with my existing equipment. It's now possible to share the Windows 7 machine's Internet connection at the same time as the AirPort's drive. An additional challenge was to avoid having to manually reconfigure every WiFi client, but it turned out that's also possible.
For the sake of posterity, here's the guide! :-)
Background
My setup is based on an AirPort Extreme access point WiFi network. The AirPort shares a hard drive, and a number of Windows clients connect to the network. One of these, a Windows 7 machine, has a USB mobile broadband dongle which I also wanted to share with the network.
Previously I got this to work, but whatever configuration I came up with required manual reconfiguration of each network client, one way or the other. This was not ideal. Also, Internet Connection Sharing (ICS) defaults to IP address .1, the same one to which the AirPort defaults. This needed manual reconfiguration of the ICS host each time ICS was turned on.
Solution
THIS SOLUTION REQUIRES THE AIRPORT TO BE IN EITHER DHCP + NAT OR DHCP MODE. IF FOR SOME REASON YOU DEPEND ON THE AIRPORT BEING IN BRIDGE MODE, UNFORTUNATELY THIS SOLUTION WILL NOT WORK FOR YOU.
1. Do the AirPort's non-Internet configuration
On the AirPort Utility main screen, click the AirPort button, work through all the tab sheets and configure the items as you require. Do the same for the Printers, Disks and Advanced buttons. Once that's complete, we can do the Internet configuration.
2. Set the AirPort to DHCP & NAT
This is needed so you can choose the AirPort network address in the next step (but you can still set a different Connection Sharing mode later on, see step 7). For now, set Connection Sharing to Share a public IP address:
3. Set the AirPort to the same network address as ICS
Set the AirPort to the network address used by ICS, namely 192.168.137.xyz. Note that the AirPort does not allow a beginning address below 2:
4. Obtain the AirPort's MAC addresses
These will be used in the next step. Record both the Airport ID as well as the Ethernet ID:
5. Fix the AirPort IP addresses to prevent clashing with ICS
The AirPort will default to the same IP address as ICS, namely 192.168.137.1. To prevent this, assign different addresses to the AirPort with DHCP reservations. Underneath DHCP Reservations, click the '+' and create reservations for each of the AirPort and Ethernet interfaces. You can use other values, but to be consistent with the rest of this guide use 192.168.137.253 for the AirPort interface and 192.168.137.254 for the Ethernet inteface:
6. Distribute ICS configuration data with AirPort DHCP
Yes, you read right! This is where the magic happens. To avoid having to manually configure each WiFi client for the additional ICS network, that configuration data is included with the AirPort DHCP configuration. Assuming you've stuck to the numbers above, configure the TCP/IP screen exactly as shown. .254 is the AirPort router (and also its Ethernet interface) address, and .1 is the IP address that Windows configures for the ICS interface:
7. Set AirPort Connection Sharing
If your AirPort is sharing a connection on its WAN port, choose Share a public IP address, otherwise choose Distribute a range of IP address. In the latter case the NAT tab disappears:
8. Save your AirPort's configuration
Your AirPort configuration is now complete and you can push the Update button. Once your AirPort has rebooted, its network will be fully operational.
9. Ensure your Internet sharing port is correctly configured
ICS involves two network ports on the sharing computer - the shared Internet-connected port, and the sharing port connected to your local network. When you enable ICS on the shared port, Windows will temporarily configure the sharing port's settings to fit in with the scheme above (it will get the fixed IP address 192.168.137.1). When ICS is disabled, Windows will restore the default configuration of the sharing port. The purpose of this step is to ensure that the default sharing port configuration is correct. When ICS is inactive the sharing port needs to obtain an IP address automatically from the AirPort via DHCP. Right-click your chosen WiFi sharing connection > Properties > Networking > Internet Protocol Version 4 (TCP/IPv4) > Properties and configure as follows:
10. Share your Internet connection
Right-click the connection you want to share > Properties > Sharing and configure as follows (you may find the other settings also useful but they're not covered here):
And now you're ready to enjoy your local network services as well as your shared Internet connection, all on your Airport Extreme network - without having to lift a finger to configure network clients!
Windows Network Profiles
If you use Windows network profiles to distinguish between Home, Work and Public networks, here are some additional notes you may find useful.
As Windows recognises networks, it applies their assigned profiles which add or remove sets of restrictions wrt. network discovery, file & printer sharing and so on. The amount of ICS configuration data that can be included with AirPort DHCP information is limited and in some instances this interferes with how Windows recognises networks.
It seems that Windows recognises a network once a default router has been configured for it. Following are some known scenarios with fixes for each, based on that principle.
1. WiFi network not recognised
When ICS is enabled, on the ICS host Windows configures only an IP address for the sharing port and no default router. When ICS is disabled, on the ICS host this port receives its configuration from the AirPort via DHCP, as do all other WiFi clients. This DHCP configuration includes router information for the ICS network, but again no router information for the AirPort network itself.
Therefore the AirPort network is not recognised on any Windows machines on the WiFi network, under any circumstances.
To make Windows recognise the AirPort network, execute the following from the command line with Administrator privileges:
     route -4 add 0.0.0.0 mask 0.0.0.0 192.168.137.254 metric 3000
This metric is high enough to ensure that "real" default routes get preference over this one.
If you want to remove the above routing table entry, execute the following from the command line with Administrator privileges:
     route -4 delete 0.0.0.0 192.168.137.254
For convenience I created shortcuts for these two commands.
2. WiFi network recognised, unrecognised network remains
Once you've added the route above to make Windows recognise your AirPort network, when ICS is disabled Windows will probably identify TWO networks on your WiFi adapter:
     Unidentified network, your_AirPort_network
Windows now recognises your AirPort network, but the DHCP router entry for the inactive ICS network makes it think there must be yet another network. However, the 192.168.137.1 ICS port is not present when ICS is inactive, so Windows reports the inactive router entry as an unidentified network.
If you want to remove this unidentified network/router entry, execute the following from the command line with Administrator privileges:
     route -4 delete 0.0.0.0 192.168.137.1
On your Windows ICS host, removing this entry can cause no harm.
However, on network clients where you removed this entry and subsequently want to share your Internet connection from again, you'll need to add it back.
To restore the ICS router entry, execute the following from the command line with Administrator privileges:
     route -4 add 0.0.0.0 mask 0.0.0.0 192.168.137.1 metric 1