Multi subneting using DHCP

You can use a single dhcp server for more than one subnet. However, you need to incorporate these different scopes into a superscope.
Secondly, you need to configure a dhcp relay agent on all subnets that do not have a dhcp server. Make sure the relay agent is not on the dhcp server or it will not work.

DHCP servers, by default, can only provide addresses to workstations that they can see broadcasts from. If your DHCP server happens to be doing the routing for your network, then it "sees" all subnets, and you just have to define a scope for each of the subnets.

In most cases, the DHCP server ISN'T the router, but you still define the scopes for each of the subnets the same way. Again, the server can only service packets in can see, either directly or forwarded.

How are your multiple subnets connected in the first place? If they have a Cisco router, you can use the "IP helper-address xxx.xx.xx.xx" on the distant subnet so that their requests will be routed to the DHCP server; who then forwards the request back to the router that will THEN send the reply back to the remote client. You could also set up a server running DHCP relay on each of the remote subnets.