Your switch, router or hub determines the IP, if you did no configuration out of the box.
Youll want to set a static IP for each of the systems sharing the internet connection.
Then youll have to set up a port forward at the router to forward all connections to port 22 onto the actuall IP of the Linux box you want to connect to when you're not at home.
The reason you have to port forward is that you can't connect to a 192.168.*.* address (which is what your switch or router divies out to your computers)
192.168 is non routable. So you would enter the ip of the MAIN ip that the switch is grabbing from your cable modem (or whatever device) then when it gets a connection at that address and realizes its on port 22, it will forward it to the machine that you tell it to.
Your linux box says hello, your ssh shakes hands. password: