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Thread: DHCP page 95

  1. #1
    Join Date
    Aug 2011

    Default DHCP page 95

    figure 3.3 shows the DHCP client four step process

    can anybody explains why does the DHCP client broadcast a request after receiving an offer reply from the DHCP server

    it found the DHCP server so why cant it just send a unicast message to it ?? without disturbing everyone else on that network, the server communicates to it with a unicast message that sounds right

    also on page 123 on the last line it says
    DHCP client request is a good example of how a unicast works ???

    now thats got me even more confused

    as per the four step process DHCP client sends out a DHCP discover broadcast and after getting a reply from DHCP server it again send s a broadcast as i have mentioned above,

    DHCP client seems to be dealing only in broadcasts but the DHCP server is dealing only in unicasts which i feel is smart

    Last edited by aneksadh; 08-31-2011 at 02:58 AM.

  2. #2



    There can be more than one dhcp server that responds to the clients discovery broadcast.

    The offer/request terminology does sound backwards, but whats happening is that the client request (where that client accepts only one of the offers) is the same as it saying "ok I'll take your offer, please send my network ip configuration" - and the reason that it is a broadcast and not a unicast is, as I understand it, so that it can at the same time in that broadcast inform the the other servers, if there were any other making an offer, that the client has chosen that one particular server and they can now withdraw their offers and return the address to their dhcp pools.

    In the broadcast then, there is a field that indicates the one particular dhcp server, whos offer the client has chosen. I don't know the name of the field.

    Also remember, at this point, the client is still waiting for an address. Not sure if that has anything to do with sending a broadcast as opposed to a unicast though. After all it does have mac address, so it seems to me that it could. I think the real reason is so that it can efficiently let all the dhcp servers know at once of its decision, and the one chosen can then respond with the final DHCPACK, while the others withdraw their offers.

    Someone else may have a better, more complete way of explaining the process.

    Now that I think about, I think I have read somewhere that it actually CAN be a unicast request, but ONLY if there is one dhcp server.

    multiple servers: broadcast / unicast / broadcast / unicast
    one server: broadcast / unicast / unicast / unicast

    Can someone confirm this, or set me straight?
    Last edited by ciscodaze; 09-01-2011 at 01:21 AM.

  3. #3
    Join Date
    Aug 2011


    thanks kevin,

    you mentioned a very important point which i could not have imagined while reading the book that the client sends a broadcast to all the dhcp servers which might have sent it an offer, i feel this point could have been explicitly mentioned in the book, i took it as just one dhcp server in the picture and as per the basic logic of broadcast and unicast which was taught earlier in the book i feel everyone would have raised an eyebrow on reading this 4 step process


  4. #4


    Everything in my last post is correct, except for the last part.

    Did some research and apparently the clients "request" is always a broadcast.

    However if there are several dhcp servers (some on different subnets) and the the local router is acting as a dhcp relay, (the only way to get the broadcast to cross the router), it will use unicast to reach the other dhcp server(s).

    Ok, Ill shut up now before I confuse everyone.
    Last edited by ciscodaze; 09-01-2011 at 04:02 PM.

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