Your DHCP manager may support MAC Address Access Control Lists, giving out IP addresses to known, authorized MACs
while denying requests from everyone else. This method is slightly more scalable -- the same ACL works no matter which switch port or AP a given laptop is using. However, it is still vulnerable to MAC spoofing.
Many new Ethernet switches and wireless APs support 802.1X Port Access Control. 802.1X is designed to overcome MAC spoofing by dynamically enabling/disabling a LAN port based on something more than MAC address. Specifically, the wired or wireless laptop will be challenged for credentials and must authenticate before network access is granted. To use 802.1X, you will need an Authentication Server that supports 802.1X, but you can probably run that on one of your NT servers and leverage your Windows workgroup and domain to authenticate LAN users. To learn about 802.1X, check out searchNetworking's Wireless Lunchtime Learning Access Control lesson.
Finally, you can also control access to your NT servers at a higher layer. For example, you could put a small business firewall between the servers and all LAN stations, requiring users to log in at the firewall to gain access to the server subnet. Or you could require domain login when workstations access individual applications and shared files/printers offered by your NT servers. Ideally, you should consider creating a layered defense by controlling access to both your LAN and your network/applications.
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