Introduction to DHCP

Lessons Discussion
19

Hi Saranya,
This topic can be a little bit confusing because there are two different layers that can perform broadcast or unicast - Layer 2 and Layer 3.

Here is a summary of what happens at each layer for each phase:

Phase      Layer 3      Layer 2
Discover   Broadcast    Broadcast
Offer      Broadcast    Unicast
Request    Broadcast    Broadcast
Ack        Broadcast    Unicast

Note:
Layer 3 broadcast = 255.255.255.255
Layer 2 broadcast = FFFF.FFFF.FFFF

You may notice that layer 3 is always broadcast. This is because the whole purpose of DHCP is for the client to establish its layer 3 address, which will not happen until the conclusion of DORA. Additionally, you may notice that all communication from the DHCP server at layer 2 is unicast. The reason for this is because the DHCP server obtained the client’s MAC address when the client sent out its initial Discover message.

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