Nas with dynamic IP address

I have freeradius with mysql and chillispot. The freeradius works with a static ip address, but most of the Nas are behind dynamic public IP adress as they are connected with Xdsl. Does it possible to check Nas validity not with their IP adsress but with their Mac Address. I actully set up a Nas in sql with IP 0.0.0.0/0. it works, but i think it s not safe. Thanks in advance for your help.