The DHCP Server Callout DLL helps to filter out DHCP requests based on MAC address.
When a device or computer tries to connect to network, it first tries to obtain an IP address from the DHCP Server. The Callout DLL (read: hook-in DLL) also works and should continue to work on Windows Server 2008.
The DHCP Server Callout DLL checks if this device MAC address is present in known list (text file) of MAC addresses configured by administrators.
If it is present, the device will be allowed to obtain an IP address or the device requests will be ignored based on the action configured by administrator.
MAC address based filtering will allow network administrators to ensure that only a know set of devices in the system are able get ip address from DHCP Server. This DLL will help administrators to enforce additional security into network.
After installation, both the DLL (MacFilterCallout.dll) and the installation/configuration instructions (SetupDHCPMacFilter.rtf) are available under %windir%system32.
Additional Information : DHCP Team Blog
Source : TechLog