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Multiple DHCP Scopes - How does that work?

So, we have run out of IP addresses at work and are going to change our IP scheme completely.  No big deal, I know how to do that.  However, our Windows Server 2012 handles DHCP for two separate networks and I am confused as to how it works.  Obviously we have a router (Palo Alto brand) that routes between the networks.  I can see the two scopes in DHCP, but how does our server know where the request is coming from to determine what scope to send an IP back to the requester?  

The reason I am asking is because I have to change these scopes with the IP change and I am worried it will break DHCP for our second network.  If it does, I want to know how it works so that I can try to fix it.  Any advice on how this works or what to watch out for?
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Tim Phillips
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Tim Phillips
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Schuyler DorseyCommented:
To answer your latter question.. when you create a new vlan and a new DHCP scope on a server.. and that new segment is separate from the DHCP server by a layer 3 router, the layer 3 router has to be configured with an IP helper address. So DHCP requests it receives, it will forward those to your DHCP server.
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Tim PhillipsAuthor Commented:
I get that (I recall IP helper addresses from my Cisco days), but how does the DHCP server know which scope to assign to that router's relayed request?  As far as I can tell, the scopes on the server are identical in settings except their IP networks.  Let's say I had 10 scopes, how would my server know which one to use for what?
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Schuyler DorseyCommented:
I believe it is based on the originating subnet. So when the router forwards the request to the DHCP server, the packet contains the originating network segment. The DHCP server compares the the originating ip segment against its scopes.
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Tim PhillipsAuthor Commented:
Oh, ok, so the IP Helper packet/function may have something in it that says, "I have a request for an IP and it came in on x.x.x.x network."  Then Windows matches x.x.x.x to a scope and issues out that IP back to the IP Helper address.  Cool that makes sense.
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jburgaardCommented:
I assume you do not have more IP-addresses assigned to the DHCP-server.
IP Helper address pointing to DHCP-server on the routing device let DHCP-packets traverse from one vlan to other. Without a setup  like this the broadcast-packets (such as DHCP) would not make it  between client-network and server-network.
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Tim PhillipsAuthor Commented:
I double checked, but yes our server only has the one IP address on it's NIC.
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