Editing Dynamic Host Configuration Protocol (section)

==Relaying==
In small networks, where only one IP subnet is being managed, DHCP clients communicate directly with DHCP servers. However, DHCP servers can also provide IP addresses for multiple subnets. In this case, a DHCP client that has not yet acquired an IP address cannot communicate directly with a DHCP server not on the same subnet, as the client's broadcast can only be received on its own subnet.

In order to allow DHCP clients on subnets not directly served by DHCP servers to communicate with DHCP servers, DHCP relay agents can be installed on these subnets. A DHCP relay agent runs on a network device, capable of [[routing]] between the client's subnet and the subnet of the DHCP server. The DHCP client broadcasts on the local link; the relay agent receives the broadcast and transmits it to one or more DHCP servers using [[unicast]]. The IP addresses of the DHCP servers are manually configured in the relay agent.
The relay agent stores its own IP address, from the interface on which it has received the client's broadcast, in the ''GIADDR'' field of the DHCP packet.
The DHCP server uses the GIADDR-value to determine the subnet, and subsequently the corresponding address pool, from which to allocate an IP address.
When the DHCP server replies to the client, it sends the reply to the GIADDR-address, again using unicast.
The relay agent then retransmits the response on the local network, using unicast (in most cases) to the newly reserved IP address, in an [[Ethernet frame]] directed to the client's MAC address.
The client should accept the packet as its own, even when that IP address is not yet set on the interface.{{Ref RFC|2131|rp=25}}
Directly after processing the packet, the client sets the IP address on its interface and is ready for regular IP communication, directly thereafter.

If the client's implementation of the IP stack does not accept unicast packets when it has no IP address yet, the client may set the ''broadcast'' bit in the FLAGS field when sending a DHCPDISCOVER packet.
The relay agent will use the {{IPaddr|255.255.255.255}} broadcast IP address (and the clients MAC address) to inform the client of the server's DHCPOFFER.

The communication between the relay agent and the DHCP server typically uses both a source and destination UDP port of 67.