TCP/IP networks are interconnected by routers that connect network segments (subnets) and pass IP packets between the subnets. As mentioned earlier, one of the major components of the DHCP specification is the DHCP protocol for communications between DHCP servers and clients.
A DHCP server can provide IP addresses to clients in multiple subnets if the router that connects the subnets is an RFC 1542-compliant router. RFC 1542 specifies the DHCP/BOOTP relay agent. If the router cannot function as a relay agent, each subnet that has DHCP clients requires a DHCP server.
A relay agent is a program used to pass specific types of IP packets between subnets. A DHCP/BOOTP relay agent is simply a hardware or software program that can pass DHCP/BOOTP messages (packets) from one subnet to another subnet according to the RFC 1542 specification.