A simple checkbox and a menu allow you to share your internet connection (no matter which interface you get it on) with any other interface. (theoretically) But the guts of this is a little confusing. This page http://www.manpagez.com/man/8/InternetSharing/ describes InternetSharing, a process which reads the property list com.apple.nat.plist stored in the /Library/Preferences/SystemConfiguration.

First question: why is the airport singled out with its own special entry in the plist? I understand the entries for primary interface (the source) and the sharing devices (the targets) But who cares if airport is enabled in this plist if you are setting up a nat between ethernet and bluetooth for example.

Second question: why does it allow sharing of ethernet connection with ethernet connection and then give you a warning that your isp will hate you? (it also allows fire wire with firewire) it doesnt allow bluetooth-bluetooth or airport-airport=> the menu entries for self reference disappear.

Third question: How do you change the default network when you share the ethernet through your airport? InternetSharing configures the IP addresses for non-AirPort interfaces starting at 192.168.2.1 and the SharingNetworkNumberStart property allows you to change that. The AirPort interface by default is assigned 10.0.2.1. How to change?

Fourth question: If you only have one ethernet jack on a macbook how could you possibly have more than one interface?. InternetSharing configures the IP addresses for non-AirPort interfaces starting at 192.168.2.1, walking up by one class C network (subnet mask 255.255.255.0) for each subsequent interface i.e. 192.168.3.1, 192.168.4.1, 192.168.5.1, and so on. Is this to address the possibility of a macpro or an xserve with multiple nics?

Tnx to the guru who shares his/her(?) knowledge.If it is a her I'd REALLY to meet her.