What can the AirPort Extreme WAN port be used for?
My network at home is made up of two airport extremes, on upstairs and one downstairs. They are connected by my in wall ethernet. The airport extreme I have upstairs is connected to my modem via it's WAN port. Then I have my in-wall ethernet connected to a LAN port on the airport extreme upstairs. On the airport extreme downstairs the it's connected to the in-wall line via it's WAN port. I have always thought that this is an acceptable setup thinking that the WAN port on airport devices acted as a sort of "input" and that the LAN ports were "outputs." But as I am learning more about networking I'm not sure if I have this set up right.
So I guess my question is:
Is the WAN port on AirPort devices intended for connection to a modem and a modem only or can it be used for other purposes such as how I am using it?