A DHCP relay agent is a small program that relays DHCP/BOOTP messages between clients and servers on different subnets. To support and use DHCP service across multiple subnets, the router(s) that connect each subnet must support DHCP/BOOTP relay agent capabilities.
By default DHCP works via broadcasting on the local subnet and therefore is limited to being able to provide IP addresses for devices on its subnet only. In modern day networks this is a serious limitation as it would mean that for every subnet you would need a DHCP server – imagine the management overhead!
So how can we configure a DHCP server in one subnet to respond and provide IP addresses to clients in multiple other subnets? The answer is to use a DHCP Relay Agent.