Configuring DHCP Server and IP Helper on Packet Tracer
any people wonder how this works but the answer is quite simple. It's called an IP Helper address. DHCP IP Helper addresses are IP addresses configured on a routed interface such as a VLAN Interface or a routers Ethernet interface that allows that specific device to act as a "middle man" which forwards BOOTP (Broadcast) DHCP request it receives on an interface to the DHCP server specified by the IP Helper address via unicast.
To configure an IP helper address you'll use the ip helper-address a.b.c.d in interface configuration mode on the interface that is connected to the broadcast domain in which you wish to provide DHCP IP addresses. For example, a VLAN interface or an Ethernet interface on a router connected to a Cisco switch or segregated by a layer 2 VLAN.
In this lab R1 and R2 are placed separate VLAN's and you will create DHCP pools for each VLAN on R1 then configure an IP Helper address on SW1′s VLAN20 interface connecting to R2 VLAN to ensure that devices on that Ethernet segment can receive DHCP IP address from the DHCP Server (R1). You will test the DHCP and IP Helper configuration using R2 as a simulated host PC.
www.networkingforbeginners.net