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DHCP

Posted on 2004-09-06
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Last Modified: 2012-10-17
How does a DHCP server, configured with 10s of scopes, knows which scope to use to lease an IP address to a DHCP client, which may be on a different subnet.

Thanks in advance.

Hamid


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Question by:htavakko
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Chireru earned 2000 total points
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DHCP uses broadcast in order to work.  (the machines broadcast a request because they have no proper IP yet, and the DHCP server broadcasts back an offer, etc).  If they are not on the same network, the broadcasts will not get through.  (broadcasts will not be routed between subnets or different networks).

Therefore, there is an in-between service, usually running on the router between the networks called a "DHCP agent".  The DHCP agent catches any DHCP broadcast requests, and forwards them unicast to the DHCP server, which tells the DHCP what subnet they are on.

So, if the host is on one of the networks the DHCP server is attached to, it knows because it's connected to them.  If it's not directly connected, the DHCP relay agent will tell it what network it's connected to. (derrived by the agent's IP address).
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