ok, all we need if the IP right?
set a CGI script to log the IP of all who visit...
then schedule a task to GET the cgi url, this can be done pretty easily with a number of tools (the browser, etc..) - almost all networks allow HTTP traffic

here is an example:
serverside cgi:
now, we can either put a timer in the perl script - or use another scheduling tool (like those offered with most OS's )
this is the same kind of setup i have on my home box. that way all i have to do (if my IP changes) is goto
http://our.server/blah.txt and looks for the latest entry...