Traditional DHCP requests from a client must find a DHCP server using a Layer 2 broadcast (thus in the same subnet). So, if the router is not able to function as a proxy for the broadcasts, it would be necessary to put a DHCP server on every network segment where such service is needed. Thanks to the ip-helper command, when a client sends a DHCP request packet to a broadcast address, the router replaces the source address with its own IP address for the interface that received the request. And it replaces the destination with the address specified in the ip-helper command. Thus, as soon as a pool is configured for the given subnet, the DHCP server can retrieve an address for a host that is not in the same subnet.
FastIron FCX that runs FCXR07100a.bin (Router)
FastIron FCX that runs FCXR07100a.bin (Router that acts as a DHCP Server)