I have configured a DHCP server on Linux. My goal is to map client IP address with their MAC address in such a way that no "unregistered" client machine gets a response from the DHCP server. That is, I would like to give them Internet access one by one through IP and MAC binding.
As noted by Warren and Shawn, your question seems to imply that preventing address assignment to unregistered machines is intended to keep them off the net.
You cannot increase security this way as an machine can either:
More generally, you cannot enforce exclusion at the IP layer as it has no mechanism designed to do this. If you really need to enforce exclusion at the Internet Layer you need to use something like IPSec which is far more complicated than DHCP.
The dhcp.conf file has the ability to specify group parameters. In this group you can specify specific hardware addresses.
From the man page: