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Let us say there are two Networks: A (10.100.90.0) and B(192.168.1.0). Network A has a DHCP server.

We know that when multiple devices are connected to network A, the devices get the IP address from the DHCP server connected to network A (e.g. 10.100.90.1,100.100.90.2 etc.).

Let us say we have connected to a (new) switch to network A. The devices connected to this switch get IP addresses from the DHCP server in network A.

Network Configuration

As we see in the diagram, there is a router in between two networks. I am using a load balancing router. I want the Laptop 0 to get its IP address from the DHCP server connected on the network A side (ignore the server on Network B side; it does nothing).

Edit:

I think I should change the way I ask the question!

Can I use a (external) DHCP server instead of the router to assigning the IP addresses to the devices connected to it?

Update-1:

The Ip-Helper address command works!!! But the DHCPREQUEST Packet is being rejected by the DHCP Server. No DHCPOffer is being sent back to the client?!

Update-2:

I added a pool of addresses for network B in DHCP server and laptop 0 was able to get an ip address from the DHCP Server! :-)

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Your DHCP server can serve addresses for both LANs, but the configuration of the server is off-topic here.

If you want Laptop 0 on Network B to get an address from the DHCP server on Network A, you need to put in a helper address on the router interface for Network B:

interface GigabitEthernet0/1 ! assuming this is the interface for Network B
 ip helper-address 10.100.90.1

Edit:

Yes, you can use an external DHCP server. I detailed how to do that above. This is how DHCP is done in large deployments so that the DHCP servers can be consolidated. You can have multiple helper addresses to servers in separate places for redundancy, and the hosts will use whichever responds first.

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  • It works!!!! But the DHCP REQUEST Packet is being rejected by the DHCP Server... It doesnt send back any DHCP OFFER Packet..... – Vineeth Penugonda Apr 2 '16 at 0:00
  • Most likely, you don't have a scope for that network configured on the DHCP server. – Ron Maupin Apr 2 '16 at 0:16
  • Added a scope and it worked perfecctly.... What if I want the DHCP to assign the ip address as 10.100.90.5 (any 10.100.90.x/IP address of network A) to laptop0(it is connected in network B) then? – Vineeth Penugonda Apr 2 '16 at 0:19
  • That won't work. Even if it did, the router couldn't handle it, and the device wouldn't be able to get off Network B since its gateway would be on the other side of the router. – Ron Maupin Apr 2 '16 at 0:26

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