as I can understand , the Ip helper address is used to convert the broadcast requests coming from the LAN ( like dhcp request ) to unicast packet and send it to the specified server ( Like DHCP server )
the problem is when the router doing that it can route the packet toward the server , because it has the server IP in the routing table, but in case of DHCP , how the server route the repy back top the host ?
the host has no IP and when the router recive it how he will send it to the specific dest PC , in normal operation the router send arp request asking for the MAC address for this IP , but here the PC has no IP , so it will not answer , and if the router will send it as a broadcast to the LAN , who inform the router to do that?
does the router i nthis case recognize that is a DHCP reply and need to be sent as Broadcast ?
You are correct that the router receives the DHCP request which is a broadcast and forwards it as a unicast to the DHCP server. There is a field in the DHCP request for the Gateway Interface address and the router has filled in that field. This is how the DHCP server can send the response back to the router. And since the DHCP request and the DHCP response have the MAC address of the requesting station the router is able to forward the response.
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