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We have DHCP setup at the office on one of our Windows Server 2008 boxes. Everyone that has a wired connection goes through our router at 10.10.10.1 and is assigned an address from our DHCP server at 10.10.10.20. The DHCP scope is setup to assign IPs for 10.10.10.50-200.

Then, we added a wireless network using the same scope and we ran out of IPs. The wireless router has an IP of 10.10.20.1 and has DHCP running on it that assigns clients an address of 10.10.20.50-200. This is fine, except that I want to manage the DHCP on the Windows 2008 Server at 10.10.10.20. If I turn off the DHCP on the wireless router how do I guarantee that all wireless devices will get assigned an IP from the proper scope?

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1 Answer

up vote 6 down vote accepted
  1. Create a separate subnet for wireless on your router/L3 switch and set up the necessary routes to/from it.

  2. Don't use the wireless router as a router, use in L2 bridged mode. This will make it into an AP. You don't want to route with that device, let your core L3 switch/router do the routing.

  3. Put a DHCP forwarder entry (IP Helper in Cisco speak) that points at your DHCP server on that router/L3 switch.

  4. Create the scope for the 10.10.20.0/24 subnet on the DHCP server.

If you have a flat L2 network structure internally, you won't be able to do this. You'll have to create a separate subnet for wireless. It's pretty common, though, so don't think it's unusual to do.


In the event that you have all SOHO gear in your office and don't have a real capable router that can handle vlans/multiple subnets/ip helpers/etc, just make your 10.10.10.0/24 into a /23 or /22 and broaden your scope. It would all be on the same broadcast domain, which is less than idea, but perfectly doable at that size. You don't really need to start separating traffic until you have a packed /21, in my experience.

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What do you mean by "create a separate subnet". Does that give it a different subnet mask? –  Micah Aug 1 '12 at 14:50
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I think you might need to read this excellent Q&A about subnetting. 10.10.10.0/24 (your current wired network) is a subnet. Your wireless scope of 10.10.20.0/24 is also a subnet. For traffic on different subnets to reach their destination, there have to be routes in place. This is what a router (or L3 switch - a specialized router for internal core networks) does. You need to define these subnets on your core routing device as a first step to getting what you want. –  MDMarra Aug 1 '12 at 14:53
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