What is IP Helper? why is it used?
Let’s say you have a network with two subnets: Subnet A and Subnet B. Subnet A has a DHCP server that is responsible for assigning IP addresses to clients on that subnet. However, you also have clients on Subnet B that need to receive IP addresses from the same DHCP server.
To achieve this, you can configure a router that connects both Subnet A and Subnet B to act as a DHCP relay agent. You would configure the router interface that connects to Subnet B with an IP helper address that points to the IP address of the DHCP server on Subnet A.
When a client on Subnet B sends out a DHCP broadcast message requesting an IP address, the router receives the broadcast and forwards it to the DHCP server on Subnet A using the IP helper address. The DHCP server then responds with an offer of an IP address, which the router forwards back to the client on Subnet B. The client on Subnet B can then accept the offer and receive an IP address from the DHCP server on Subnet A.
In this way, the IP helper address allows DHCP clients on one subnet to receive IP addresses from a DHCP server on another subnet
When user sending DHCP request to get IP, at that time it’s generating broadcast traffic in within VLAN. Router or L3 switch is helping to reach it till DHCP server by help of IP helper, and it help’s to convert boradcast traffic to unicast. IP helper need to config on router/L3 and it will be a pool IP on DHCP server. Without IP helper DHCP can’t work.