Change the default subnet for Internet Sharing

[Original Article: Change the default subnet for Internet Sharing • comment by: glusk]
Apparently, Yosemite (10.10) now needs a bit more info to start InternetSharing.

SharingNetworkMask
255.255.255.0
SharingNetworkNumberEnd
192.168.0.20
SharingNetworkNumberStart
192.168.0.0

Because of cacheing, you should use defaults write:

sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.nat NAT -dict-add SharingNetworkNumberStart 192.168.0.0
sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.nat NAT -dict-add SharingNetworkNumberEnd 192.168.0.20
sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.nat NAT -dict-add SharingNetworkMask 255.255.255.0
I hope someone more familiar with writing plists can improve this, but it worked for me as written. …

Source: Mac OSX Hints