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Forum Replies

  1. 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 clien

    ... Continue reading in our forum

  2. Dear Rene/Andrew,
    Thank you for this great lesson. Mr Andrew with reference to your reply # 27608 above particularly this point " 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.", I am still confused on where broadcast happens and where unicast happens. From the Wireshark captures above I do not see Unicast happening anywhere. Even for Offer and Ack from the server the dest mac ad

    ... Continue reading in our forum

  3. andrew
    Could you please explain why OFFER message from server at layer 2 is broadcasted while i think it should be unicast in nature as DHCP sever has already got the client MAC address in client DISCOVERY message.

  4. Hello Samit

    This is an excellent question and it shows that you’re thinking deeply about the subject. It is true that the DHCPOFFER when sent can technically be sent using a unicast MAC address since the MAC address of the host making the request, and thus the destination of the DHCPOFFER frame, is known. However, some operating systems and NIC drivers don’t always use this logic when operating DHCP.

    Some client implementations are unable to receive such unicast frames until the implementation has been configured with a valid IP address. Remember, when we en

    ... Continue reading in our forum

  5. Hello Swapnil

    DHCP can provide a multitude of information to hosts. The most common implementations include IP address, subnet mask, default gateway and DNS server. There are many more elements that DHCP can offer and these are called DHCP options. Some of the most common include NTP servers, log servers, cookie servers, interface MTU, default TCP TTL, NetBIOS name server and IRC chat server to name just a few.

    These options are indicated using an option number. DHCP option numbers can range any where from 0 to 255. Some of these numbers are standard vendo

    ... Continue reading in our forum

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