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In 'Request' part of the DORA process, IP and MAC address of the DHCP server is known to the client (as the IP datagram sent in Offer has this information). So if the destination is known, why is the request still broadcasted to every machine on the network?

Similarly in 'Ack' part of the DORA process, the IP address of the client is now successfully set inside it and known to the DHCP server. Why does it still broadcast the message?

Paras Gera
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2 Answers2

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As several DHCP servers can respond to a clients DHCP Discover message, the broadcasted DHCP Request message from the client contains information on which IP address from which server the client chose to accept. The other servers then have the opportunity to immediately withdraw their offers instead of letting them time out.

ErikE
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  • This is a nice explanation about the request message. But what about ack? Why is that too broadcasted? – Paras Gera Aug 18 '15 at 09:38
  • The server can either unicast or broadcast the DHCPACK, depending on the state of the DHCP negotiation. The different use cases are detailed in RFC2131: https://tools.ietf.org/html/rfc2131 – ErikE Aug 18 '15 at 10:46
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as you are asking about offer, request and ack let me explain one by one , first about offer it has to be broadcast because the client is not having any ip address. Now about request the client is having the ip address of the server so why thi message is broadcast the answer is that what if there are two or more servers who have responded to the client and gave him the ip, so to let them know that I have choosen this client the request message is broadcast. Now ack has to be broadcast because client yet has not taken the ip adresss which the server gave him only he has chosen the ip adresss. now client will send him ack packet "that you can have this ip".

aamir azam
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