23 In a subnetted environment, how does the DHCP server discover what subnet a request has come from?
DHCP client messages are sent to off-net servers by DHCP relay agents, which are often a part of an IP router. The DHCP relay agent records the subnet from which the message was received in the DHCP message header for use by the DHCP server.
Note: a DHCP relay agent is the same thing as a BOOTP relay agent, and technically speaking, the latter phrase is correct.