The default subnet that is used with Mac OS X when you’re using Internet Sharing (aka natd) is 192.168.2. However, you can override this value by editing the /Library/Preferences/SystemConfiguration/com.apple.nat.plist. If there is no SharingNetworkNumberStart key defined in the file then you can add one to the NAT dictionary of the property list. This can be done using the defaults command as follows (assuming a new subnet of 10.10.10):
defaults write /Library/Preferences/SystemConfiguration/com.apple.nat SharingNetworkNumberStart 10.10.10.1
