I configured a DHCP server on my Ubuntu 12.04 using the
isc-dhcp-server package. But I want my DHCP server to serve requests which are received from my Wireless Access Point and not to/from any other machine. How do I go about configuring this?
Disable any other DHCP servers on the LAN and configure your Wireless Access Point (WAP) as a wireless switch by connecting it to your existing LAN (containing your DHCP server) via the LAN ports of the WAP.
You can't practically do what you are attempting in the general case as an Access Point is a layer 2 device and will appear to the DHCP server to be no different to an ethernet device. Thus as @packets suggested you should move everything onto 1 DHCP server.
In a specific case you could set up your DHCP server to only answer for specific mac addresses - those of devices (wireless interfaces), by hard-coding each mac address into the configuration file, and not handing out IP addresses for other machines.
Alternatively you could add an interface on your DHCP server and connect the access point to that. You would then (probably/practically)need to use routing in your network, but you could select to only bind DHCP to the interface connected to the wireless access point.
[I note that as a purely intellectual excersize it is probably possible to bridge the Interfaces and do some ebtables filtering to prevent requests leaving the non Wireless AP interface instead of doing routing, but this would be a bit of a hack and quite complex to set up - and even more complex to debug. ]