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I have some doubts I can't clarify in the manual (Kurose or Tanenbaum)

DHCP use a link-layer broadcast. If we are using DHCP and to receive a response, the node should be in the same link as the DHCP server?

In the same way, the IP gateway for a sub net should be in the same DHCP server?

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2 Answers

If we are using DHCP and to receive a response, the node should be in the same link as the DHCP server?

Yes and no - something in the link-layer broadcast domain must be able to handle the request. But that doesn't mean it needs to be the DHCP server itself - lots of networking equipment has the ability to act as a relay agent, catching the broadcasted request and forwarding it on via unicast to a configured DHCP server.

In the same way, the IP gateway for a sub net should be in the same DHCP server?

I'm not quite sure what you're asking here - do you mean that the DHCP server must be the same device as the gateway? (It doesn't need to be.) Or do you mean that the default gateway for the subnet must be in the same broadcast domain as the subnet's clients? (Yes, the gateway must be in the broadcast domain, unlike the DHCP server.)

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I mean becuse DHCP use link layer broadcast, IP gateway for subnet must also be a DHCP server. It's the same to say that the IP gateway and the DHCP server are the same device. I'm not sure if that condition is necessary to the function of the system. –  taunus Nov 24 '12 at 23:59
    
@taunus No - they sometimes are the same device for convenience but there's no reason that would need to be the case. –  Shane Madden Nov 25 '12 at 0:23
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DHCP works using broadcasts which will travel between collision domains (Layer 2, switches) but not broadcast domains (Subnets, routers).

You can, however configure ip forwarders / ip helpers on the routers to do this. Even better, the DHCP sever will know which subnet the request originated from, meaning a single server can server for multiple subnets.

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But in order to receive a responde, this is a required condition that the link used by the node using DHCP should be equal as the link where is the DHCP server itself? If the node using DHCP and the DHCP server are in diferent links are we sure to receive a response from the broadcast? –  taunus Nov 25 '12 at 0:06
    
@taunus Your terminology is confusing, but I think you have some fundamental misunderstandings about how DHCP works. The initial request (Called "discovery") from the client is a broadcast, which can be specifically configured to be passed on by your routers. From then on, all communication is unicast and will be routed accordingly. –  Dan Nov 25 '12 at 10:37
    
sorry for my terminology, sometimes I have some difficulty to express myself because my native language is not english –  taunus Nov 25 '12 at 11:07
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