Here's an option to set up a server with a machine that accesses the internet via a wireless AirPort Base Station.
Go toand open: applications/utilities/AirPort Admin Utility
Click on "Configure"
Select your base station and log in to your network. This will allow you to make changes to your base Station settings.
Click on the "Network" tab and change the DHCP lease time to a something like 2 days.
Click "update"
-----here's what you did----
DHCP is a service that provides IP addresses dynamically. A simple way of explaining it is your machine starts up and "asks" your Base Station for an IP. Your Base Station "leases" a made up IP (something like 10.0.0.1) and your machine then communicates to the Base Station using that number. Your Base Station uses your "real world IP" to communicate with your ISP.
If you increase the lease time... then your Base Station will allow for more time (in this case 2 days) before it will use that number on another machine. So "computer A" joins the network as (10.0.0.1). After a few hours on the network the machine is shut down. "Computer B" joins the network as (10.0.0.2) and after a few hours it too shuts down. Your Base Station remember what machine had each of those IP addresses for (in this case) 2 days. Every unique machine will get a new IP address... and own that IP for 2 days.
This will only prevent the IP from changing, I'm not sure if it will help you with your routing issues. My recommendation is to do what I do. I have the following network setup.
WAN --> Router --> Base Station (port 1) & File Server machine (port 2).
Then my Base Station feeds my roaming laptops with their internet connection and my "Server machine" doesn't even see the Base Station. Then I set my router to direct all FTP port traffic to my "Server Machine".
Of course there's more then one way to do this.
