Question : Prevent users from connecting their personal laptops to my LAN

I have a DHCP server providing my users with IP's on logon. I noticed that if any one brings his laptop and connect to any of the ports he will be assigned an IP address automatically. I remember that there is a solution to prevent users from getting Ip's unless specified by administrator.
Any hints on that

Answer : Prevent users from connecting their personal laptops to my LAN

The one thing you could try us setting a classid on all your workstations and setting DHCP to only assign valid IP's to that ClassID