OS/X internet sharing defaults to 192.168.2.* range.
I was stumped, I was trying to hook up a second computer on the internet through ethernet. Normally it's easy enough to just turn on internet sharing. It turns out OS/X always uses 192.168.2.*, which happened to be in the same subnet as the WiFi network.
OS/X provides no way to change this as far as I could tell. Thankfully I was able to change the router's settings. If you ever get into this situation and you're not able to make this change, here's a simple guide to setup internet sharing using the console.