Connecting Clients to Remote DHCP Servers

Through the use of network broadcasts, DHCP allows client computers to locate DHCP servers on the local subnet and to obtain an IP address from that local server. However, routers by default prevent broadcasts from crossing into other subnets. Consequently, without some other added functionality, every physical subnet would need to contain a DHCP server for clients to receive DHCP service.

This added functionality can be provided in the form of RFC 1542-compliant routers, which can be configured not to block DHCP broadcasts, or DHCP relay agents, which can be configured to intercept broadcasted DHCP messages and transport them across the network to the IP address of a DHCP server.

Before passing a DHCP message on to a DHCP server, both RFC 1542-compliant routers and DHCP relay agents write their own address inside a certain field (named Giaddr) within that message. This address recorded within the DHCP message informs the DHCP server of the subnet ID of the originating subnet of the DHCP request and, consequently, of the proper scope from which to issue addresses to that subnet.