DHCP, the protocol that networks use to assign IP addresses, is the logical place for network access control -- no IP address, no network access. Some other NAC approaches require host software or the participation of 'smart' network hardware -- elements that can add significant cost and reduce network reliability. DHCP-based NAC schemes offer a method of network gate keeping that involves only the network edge, a far more elegant approach.
— Dan Golding
AccessAddressesAssign