DHCPrelays

DHCP relay is a networking function that enables Dynamic Host Configuration Protocol (DHCP) messages to be forwarded across different network segments. In typical network configurations, DHCP clients and servers must reside on the same broadcast domain for communication to occur naturally. However, when networks are segmented using routers or layer 3 switches, broadcast traffic including DHCP requests cannot traverse these boundaries.

DHCP relay addresses this limitation by acting as an intermediary agent between DHCP clients and servers located on different subnets. When a client broadcasts a DHCP request, the relay agent captures this message and forwards it as a unicast packet to one or more designated DHCP servers. The relay agent typically modifies the packet by inserting its own IP address in the giaddr (gateway IP address) field, allowing the DHCP server to determine the client's subnet and respond appropriately.

Configuration of DHCP relay involves specifying the IP addresses of DHCP servers that should receive forwarded requests. Most network devices including routers, layer 3 switches, and dedicated relay agents support this functionality. The relay agent listens on UDP port 67 for incoming DHCP requests and forwards them to server port 67.

This mechanism is essential in enterprise networks where centralized DHCP server management is preferred over deploying multiple servers across various subnets. It reduces administrative overhead while maintaining efficient IP address allocation across complex network topologies.

DHCP relay operates transparently to end users and DHCP clients, requiring no special configuration on the client side. The technology supports both DHCP discovery and request messages, ensuring proper IP address assignment and network configuration delivery to clients regardless of their physical location within the network infrastructure.