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If DHCP packet is a reply message from server, which fields are used?
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Solution:

DHCP Request:

The client, having received the DHCP Offer message, recognizes there is a DHCP server available on the same subnet.

Then it broadcasts a DHCP Request message to the server over the Ethernet network, requesting network configuration data including an IP address for itself. If more than one DHCP server responds on the same subnet and hence the client receives multiple DHCP Offer messages, it selects one of the DHCP servers and enters the IP address of the selected DHCP server in the DHCP Server Identifier (option 54) field of the DHCP Request message.

Then it informs all the DHCP servers on the subnet network about such selection by broadcasting the DHCP Request message.

Typically, all DHCP servers internally store the network configuration data (i.e. IP address for the client and other information) when they send a DHCP Offer message. So, the client broadcasts the DHCP Request message to all the DHCP servers, so that those not selected can also receive the message and delete the stored network configuration data from their memory.

The lease time is given as 1 hour. When half of it has passed (i.e. 1,800 seconds/30 minutes), the client sends a DHCP Request message to the DHCP server to renew its lease time (Note: In case of IP renewal, no DHCP Discover/Offer process is required).

Unlike in the IP address allocation/lease procedure, the client does not broadcast the DHCP Request message (Destination MAC=FF:FF:FF:FF:FF: FF, Destination IP=255.255.255.255), but unicasts it (Destination MAC=DHCP Server MAC (m2), Destination IP=DHCP Server IP (1.1.1.254)).

That is because the DHCP server and client have already known each other’s IP address.

The DHCP Request message for IP address renewal should include the IP address of the client requesting the renewal in the “Client IP Address ” field but should exclude the Requested IP Address (option 50) and DHCP Server Identifier (option 54) fields.

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