Another device on the network is using the same IP address

If you are having IP addresses assigned using DHCP and one of the devices is disconnected from the network for a long enough period of time then the router will make that IP address available to another device to use. If the first device then comes back on the network with the same IP address it had before then you'll get the dual IP address situation. This usually doesn't happen since the returning device is SUPPOSED to ask for a new IP address when it re-joins the network. In your case this may not be happening.

What I do is use static IP addresses for all the devices on my home network. In your case I think this will fix your problem.

Set up your Airport to 'reserve' specific IP addresses for the devices in your local network via their MAC addresses.

Open the airport utility, select your router, click EDIT, select NETWORK.

In the area marked DHCP Reservations: click on the + sign

Type a description of the device.

Use the pull-down menu to select "Reserve Address By: MAC address"

enter the MAC address in the appropriate line.

enter in the unique IP address for each device that you want to reserve. Make sure its in the same subnet (ie. 192.168.xx.yy)

click SAVE

Repeat the above for all your devices.

Update your airport settings.

When you're done turn off all the wireless devices for 5 minutes and then reconnect. Each should then reconnect with the same IP address each time. You can still leave the devices themselves set to use DHCP so that when you take them outsider your network, they'll request an IP address from the new network.

I hope this made sense. If not, post back and I'll calrify anything you need.

Thanks for clarifying your setup. Apparently you are using your AirPort in bridge mode and the at&t modem is acting as the router for your system. Since the at&t modem is acting as your router, it is the device that is assigning IP addresses for clients using DHCP. So it is the one that is assigning the 2 identical IP addresses. Without knowing what particular brand of DSL modem you have I can't help you much further. You may want to change the routing responsibility from your at&t modem to your AirPort. If you don't know how to do that, basically what you are doing is telling your at&t modem that there is only one device connected to it, the airport. You then let the airport create and manage your network. (router mode vs. bridge mode).

Hope this helps.

This is the exact problem I'm having. Because of my S-L-O-W performance with my AT&T service (something they could not rectify), I decided to use the AT&T modem (Model 5031MV) as a modem only while using 2 Apple Extreme Base Stations. One for one side of the house, while the other is Y-split off the [wired] telephone feed. The reason for 2 routers is due to having an old home with lathe and plaster walls and sometimes, the WI-FI signal is weak on the other side of the house.

Anyway, I also get the "...another device on your network is using the same IP address" message and would like to know how to change this.

One for one side of the house, while the other is Y-split off the [wired] telephone feed. The reason for 2 routers is due to having an old home with lathe and plaster walls and sometimes, the WI-FI signal is weak on the other side of the house.

Sorry, but are you saying that you have both AirPort Extreme base stations connected back to the AT&T modem using a "Y" splitter from the modem? Or that one Extreme is connected to the modem and an Ethernet cable is connected between that Extreme and the second one?

I'm sorry, I should've been clearer.

What I meant was that the phone jack in the wall has two "ports"; one out is going into the AT&T modem (which has an Ethernet cable going into the first Apple Extreme), and the other port (from the wall) has a VERY long cable going to the other Extreme.

Since my initial post, I looked at all the device's IP addresses and it's a MacBook laptop that is/was sharing the same IP with another device. I've since configured that laptop using DHCP manually (and input an address that it had automatically reconfigured after the error message), and it seems to be working fine now. I'm just not sure if that was the issue being that this problem has been intermittent.

What is the model number of your 2Wire modem?

edit: Sorry, never mind, I see have already provided it. Give me a minute to look it up and download a copy of the user manual so I can verify that it is only a modem and not a combination modem & router or gateway device.

Ok, is it actually the 5031NV, not MV?

Regardless, this is not a simple modem, but a gateway device. I'm just trying to understand your current network configuration and the components that you are using with it.

If I understand your connections properly, both Extreme base station are connected back to this gateway via Ethernet. They then should be configured for a roaming network. Is this the current set up? If it is, then both of the Extreme base stations should be configured as bridges so that the 5031NV is the only device that is performing the routing functions. Is this correct?

I'm sorry...yes, it's NV not MV.

Anyway, the following graphic best describes the configuration. And I was wrong, there is an Ethernet cable coming from Base Station 1 (red) that goes into the WAN port of BS 2----instead of a line coming straight from the wall jack. I did not wire it up (AT&T guys did went they came out on a call) so I'm not sure what the "Broadband" port (green) is for.

And here's a screen shot of what the Airport Utility sees.

... I'm not sure what the "Broadband" port (green) is for.

That would be your WAN connection to the Internet. Thanks for the diagram. That helps describe your current network configuration.

I am assuming that the 5031NV is performing as your "main" router. In this case are both of your Extreme base stations configured as bridges? That is, in the AirPort Utility for each device is the Router Mode set to: Off (Bridge Mode)?

Thanks, those solid lines would be telling me that each AirPort base station has an Ethernet connection to the Internet source. In this case your 5031NV gateway.

I checked the Base Stations and they are both in Bridge Mode. As I mentioned previously, since manually inputting a dedicated IP address, things have been working fine and connection(s) have been consistent. But again, this problem is intermittent so I'll keep my fingers crossed.

Thanks Tesserax for all of your help!