not finding internet connection

Another one from the downright weird zone of linux computing........

Is there any way to make ubuntu automatically re-scan for internet 
connections once its booted up and not found one?

The scenario - if no connection is present at boot up ( the modem/router 
hasn't had time to connect or such ) ubuntu caries on with booting and 
refuses to find any connection until its shutdown/restarted.


-- 
Chris