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I've got a Motorola SBG900 cable modem sitting in front of a Linksys WRT54G running DD-WRT. The DHCP server is running on the Linksys router not the modem.

I can access the DD-WRT web GUI via the router's IP address but can not access the modem's web GUI without connecting directly to it.

What configuration is necessary to be able to access the modem's web GUI when connected to the router.

Here's the setup:

  • Modem's IP: 10.0.0.1
  • Router's WAN IP: 10.0.0.2
  • Router's Internal IP: 10.0.0.3 (Gateway IP used by computers connected to the router):
  • Both the router and the modem are on the same subnet (255.255.255.0).
  • The ethernet cable from the modem is connected to the WAN port on the router.
  • Originally I believe only the router was running a DHCP server. Since the modem was recently reset, I believe it may be running a DHCP server now as well. It's hard to tell since I can't access it's GUI remotely. :)
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What addresses do you use to access your Linksys router and your Motorola modem? One of them may have to be changed if you want to access them both while they're connected together. –  Bavi_H Feb 22 '10 at 3:46
    
10.0.0.1 = modem, 10.0.0.2 = router –  user26767 Feb 22 '10 at 6:04
    
I suspect your modem and router are using overlapping subnets, but I'm confused by your setup. Here are some more questions: 1. What are the subnet masks of the router and the modem? 2. Do you connect the Ethernet cable from your modem to the WAN port of your router? 3. You said the modem isn't running a DHCP server, right? When the modem is connected to the router, what is the WAN IP of the router? Does the router get its WAN IP address from the modem or did you assign it manually? –  Bavi_H Feb 23 '10 at 2:36
    
Bavi_H: I updated the original question with the details. Thanks for your help! –  user26767 Feb 23 '10 at 15:26

3 Answers 3

As heavyd said, the address for the modem needs to be on a different subnet than the one controlled by your router.

Just to clarify, the subnet mask shows which part of the IP address is the subnet. A subnet mask of 255.255.255.0 means the first three numbers in the IP address are the subnet.

So in your setup, both the router and the modem have the subnet 10.0.0.x. When you enter the address 10.0.0.1, the router sees that address is on the subnet it controls and never sends packets for that address out on its WAN port.

So, as heavyd said, the solution is to change the router's IP address to any private IP address that doesn't begin with 10.0.0.

For example, you could change your router's IP address to 10.3.3.3 and leave its subnet mask at 255.255.255.0. In that case the router's subnet would be 10.3.3.x. Then when you enter the address 10.0.0.1, the router will see that address isn't on the subnet it controls and will send packets for that address onward to its WAN port.

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A router is used to connect 2 different subnets together. In order to get to your modem which is connected to your computer by a a router, the modem needs to be on a different subnet than your computer.

Basically, your router maps each subnet to an interface in the router and sends any traffic that it receives destined for that subnet out that interface. Since your modem is on the same subnet, all of the request destined for the modem are actually routed to the LAN, never to the WAN interface. Change your modem's IP to something like 192.168.10.1 (or anything other than 10.0.0.x) and it should work just fine.

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Why not just set your modem to bridged mode, then you won't need to access the web interface, since it will only be used to pass data from the line to the router?

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Good point. But it still doesn't solve the problem of having to plug into it directly to access the GUI! –  user26767 Feb 21 '10 at 21:59

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