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How A DHCP Request Works Across Networks

Posted on 2013-05-29
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Last Modified: 2013-05-30
I was asked a technical question by a friend something that should be basic but is much more involved: How does a DHCP request work? How does a device obtain an IP address? How does this work across multiple networks, throughout multiple routers? For example, the client requesting an IP address is in California, and the DHCP server is in New York - what is the process involved?

Thank you in advance.
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Question by:fireguy1125
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Brian Pringle earned 250 total points
ID: 39206617
First of all, DHCP works by using a network "broadcast" message, so it will not work outside of your network unless the routers have been previously configured at both location to allow broadcast traffic to be forwarded outside of the network.  This is not normal, as there is typically a DHCP server at each location.

Your computer senses that it is connected to the network and knows that it does not have an IP address.  It sends a broadcast packet to 255.255.255.255 with a return IP address of 0.0.0.0.  The packet basically says "I am here and this is my MAC address".

The DHCP server sees the packet and replies to it, querying that the computer that just sent the packet really needs an IP address.  The computer replies that it does, so the DHCP server sends out a packet stating that "the computer with this MAC address... use this IP address".

Check out the following article for more information.
http://www.thegeekstuff.com/2013/03/dhcp-basics/
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by:iammorrison
ID: 39206619
A client (PC, printer etc) broadcasts a DHCP request across the subnet (192.168.1.x for example), looking for a device or server that hosts a DHCP server role for that network. The device hosting the DHCP server role acknowledges the request and completes it, earmarking an available address from a designated pool and assigning it to the client. Is the client sure that the DHCP server is across the country? It is most likely that the two locations are on two different private networks, and there is commonly a device on the local network (like a gateway/router) that can act as a DHCP server. It is possible to use DHCP across a VPN between the offices, but it is usually much more simple and reliable (given the distance) to have a local device handle DHCP
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Assisted Solution

by:Senthil Kumar
Senthil Kumar earned 250 total points
ID: 39206826
DHCP relay Agent do this

http://technet.microsoft.com/en-us/library/cc771390.aspx
 
Also go through the below link to understand it better

http://technet.microsoft.com/en-us/library/cc940466.aspx
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Expert Comment

by:hanu_it
ID: 39206830
The below link will give clear understanding of DHCP process.

http://knowurtech.netfirms.com/networking/how_dhcp_works.html
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Author Closing Comment

by:fireguy1125
ID: 39207162
Thank you all for your help. I felt btpringle and senthil1 links provided in detail what I was looking for.
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