Re: One DHCP for 2 subnets?

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Date: 14 Nov 2006
Time: 00:23:01 -0500

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For the initial planning, forget about subnets. Think about which switches you want to be linked together. Currently all of your switches are linked so that they make up one network segment. What you want to do is change these links so the you have two network segments. This is just a matter of changing the cables between the switches. If these switches are all in the same building it should all work OK. When you have split your network into two segments, you need an IP router to connect the segments together. The router has to have one interface in each segment. You then arrange for the segments to be in different IP subnets. In the segment with the DHCP server, this is fixed by the IP address of the DHCP server's NIC. In the other segment, you do this by giving the router interface in the "new" segment a static IP in the "new" subnet. You can have just one DHCP server for both segments if the router has DHCP relay enabled. You set up a scope in DHCP for each subnet. The machines in the same segment as the DHCP server will get their network config directly from the DHCP server by broadcasting on the LAN. They will get IP addresses from the same subnet as the DHCP server's IP address. The machines in the "other" segment will also broadcast on that LAN. The router will see these broadcasts and forward the requests to the DHCP server. The DHCP server will allocate IP addresses from the correct scope, because it knows the IP address of the router interface which received the original broadcast request. Machines in the same segment as the DHCP server get IP addresses from one scope and machines in the other segment get IP addresses from the other scope, so are in a different IP subnet. How you set up the routing depends on how your current setup works and how it connects to other networks.

Re: One DHCP for 2 subnets?

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Last changed: 01/05/09