IETF RFC 3527 PDF

This document describes the link selection sub-option of the relayagent- information option for the Dynamic Host Configuration Protocol (DHCPv4). The giaddr specifies an IP address which determines both a subnet, and thereby a link on which a Dynamic Host Configuration Protocol (DHCP) client resides as well as an IP address that can be used to communicate with the relay agent. The subnet-selection option allows the functions of the giaddr to be split so that when one entity is performing as a DHCP proxy, it can specify the subnet/link from which to allocate an IP address, which is different from the IP address with which it desires to communicate with the DHCP server. Analogous situations exist where the relay agent needs to specify the subnet/link on which a DHCP client resides, which is different from an IP address that can be used to communicate with the relay agent.