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?
Tim PhillipsWindows Systems AdministratorAsked:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

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.
Tim PhillipsWindows Systems AdministratorAuthor 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?
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.

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
Cloud Class® Course: Microsoft Azure 2017

Azure has a changed a lot since it was originally introduce by adding new services and features. Do you know everything you need to about Azure? This course will teach you about the Azure App Service, monitoring and application insights, DevOps, and Team Services.

Tim PhillipsWindows Systems AdministratorAuthor 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.
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.
Tim PhillipsWindows Systems AdministratorAuthor Commented:
I double checked, but yes our server only has the one IP address on it's NIC.
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today

From novice to tech pro — start learning today.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.