How to secure LAN visiting with NIS

Hello everyone,

I've set up one LAN with NIS account verification, and limit visit to
switcher ports with MAC address binding, but I think it not so safe. If one
person use his laptop and make the same MAC address with working machine and
then connect into the LAN and set domain and NIS server, he'll get all the
visiting to the server and have the way to get data to his laptop, which is
awful. Is there any way to avoid it? I don't know how to make NIS more
secure, is there any way to set up verification server to check the legality
of machine itself? Thanks for your help!

Have a good day!

B.R.

Joffre