DHCP servicing multiple subnets


I need assistance with DHCP.
I have multiple subnets per building and want to have one dhcp server and want to give the client an ip address based on what subnet (or building) they coming from.
please let me know what additional information you need.
Thanks so much.
DHCP Server is Win2008 R2.
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SouljaConnect With a Mentor Commented:
You will put an ip helper address command with the ip address of your dhcp server on the VLAN intefaces of your 3750 switch. The dhcp server will know what scope to respond with based on the ip subet the interface that relays the dhcp request to it resides on. For example if the inteface is, the dhcp server will know to assign to reply with an address from the network.
What type of network equipment are you using? Topology? Most likely you will need to use IP Helpers to relay broadcasted dhcp traffic.
Have a look at this MS link with details of how to do this: http://technet.microsoft.com/en-us/library/cc771390.aspx
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eugene20022002Author Commented:
I know it has something to do with the IP Helper service but I have no knowledge of how it actually works.
Can some perhaps explain how a client , coming from a particular subnet or Vlan will get assigned a IP address from a particular scope?
and what is preventing the user from getting an ip address )from the same dhcp server) from another scope?

The default gateway is a Layer 3 switch (Cisco 3750 POE) WS-C3750-48P

What I really want to know is how the DHCP server distinguishes from which scope its going to assign the client an ip address from

Thanks guys
rjantonelliConnect With a Mentor Commented:
1. Use the existing DHCP Server.  It only takes one.
2. Create a correct Scope for the other subnet (no superscopes)
3. Configure the LAN Router between the subnets to forward DHCP Queries to
the DHCP Server
4. All done.

All "user" machines should be DHCP Clients with no Reservations or as few
Reservations as possible.

All Servers, Utility machines, and Network Devices should be specifically
Static (not reserved in DHCP,...not any DHCP at all).  The Infrastucture
should never have to depend on the DHCP Server being "alive" in order for
the infrastucture to exist.
x3manConnect With a Mentor Commented:
Explained here:
"A relay agent that conforms to RFC 1542 relays DHCP packets to the remote side even though they are broadcast packets. Before relaying a DHCP message from a DHCP client, the agent examines the GIADDR (gateway IP address) field. If the field has an IP address of, the agent fills it with the router's IP address. When the DHCP server receives the message, it examines the Relay IP Address field to see if it has a DHCP scope (a pool of IP addresses) that can be used to supply an IP address lease. If the DHCP server has multiple DHCP scopes, the address in the Relay IP Address field identifies the DHCP scope from which to offer an IP address lease. This process allows one DHCP server to manage different scopes for subnets. "

From: http://support.microsoft.com/kb/120932
eugene20022002Author Commented:
Thanks to everyone. Its so much clearer now. We set it up and pointed the ip helper to the dhcp and setup to ports with different vlans.
plugged in 2 laptops and they both got different ips based on the vlan they coming from. swapped it around and it worked again :-) awesome stuff.

Is there some information in the packet that lets the dhcp server know or what? is the dhcp the intelligent 1 or does the ip helper do all the work?

but otherwise the concept is working. Thanks again
thehagmanConnect With a Mentor Commented:
The dhcp client simply sends a broadcast. If it is not in the same broadcast domain (i.e. vlan) as the dhcp server, it will never obtain a reply.
However, with a helper present in the same broadcast domain, that helper will receive the dhcp discover message and relay it to the configured dhcp server. The relayed query differs in two respects: It is not a broadcast, but a unicast directed to the configured dhcp server, hence it can cross routers; also the helper adds the info about his own local network (i.e. the net of the interface that received the original request) so that the dhcp server knows which range to serve from.

In other words: "Hey, anybody, give me an address, please"
becomes "Hey, you over there, please issue an address from network so-and-so to the client having MAC address ..."
Interesting thread this one.  Can you supply any info about the commands required to setup the IP helper service ?
eugene20022002Author Commented:
Thanks to ALL the experts. Much appreciated.
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