If you are using DHCP on your network and you want to prevent rogue clients from obtaining IP addresses from your DHCP server and participating on your network, your options are simple:
- Enforce rigorous physical security. If a hacker can get through the front door and connect a laptop to your network, they can do a lot worse stuff than steal an IP address!
- Use 802.1x or IPSec to secure your existing clients from rogue clients. This won't prevent rogue clients from obtaining IP addresses however, just doing something useful with them.
- Use reservations for all your DHCP clients. In W2K3 you can use the getmac command to obtain the MAC address of a remote Windows machine if you know its IP address, and if you have a fairly small network you could write a script or batch file to run getmac for every IP address in each subnet.