I work for an ISP that uses PPPoE. I find these threads rather interesting and how complicated people are making things sound. First off, the ISP I work for has several different models of DSL modems. Some of our modems do not do any routing or assigning of DHCP at all they require DSL software on the client side (i.e. your PC such as the PPPoE client built-in to windows or in older versions of Windows a client such as RasPPPoE , enternet 300, or winpoet).

The basic modems would not have the issue with IP conflicts due to DHCP servers built-in to the DSL modem since these modems do not have any firewalls or DHCP servers built-in and cannot connect without external software. We also have a brand of modem that has "router-like" software built-in to it. When these second type of DSL modems are setup to do PPPoE they do have a DHCP Server (which can be turned off by logging into the interface of the modem our modems like most routers have a web based interface that uses a non standard Private IP to connect to it i.e. 192.168.100.254). Turning off the DHCP server on the DSL modem and hard resetting it using the pin hole reset on it causes the modem to act as a standard dsl modem (i.e. it disables the PPPoE authentication and DHCP server on the modem). If you then connect a router to the modem after hard resetting it (pin hole reset), you can then plug a router into it and let the router do the PPPoE authentication and do dhcp instead of the DSL modem.

I suspect AT&T's DSL modems probably can be reset similarly so that they act in "Bridge Mode". Bridge Mode is when your DSL modem does not do the PPPoE authenticating or DHCP functions. Most older DSL modems are bridge mode only and do not have advanced firmware to allow them to do DHCP and PPPoE. Newer DSL modems are capable of doing DHCP and PPPoE.

I suspect the AT&T DSL modems likely have a pin hole reset similar to the ISP I work for and can likely be set into bridge modem using the pin hole reset. If you reset the modem to place it in bridge mode, all this non-sense of changing the IP range should be unnesscassary.

One way you can use to determine the web admin IP of your AT&T DSL modem if it has one, is to hook up the DSL modem directly to one computer via ethernet reboot both your PC and modem to be on the safe side (make sure that the TCP/IP connection of the NIC card you are plugged into on your computer doesn't have any static IP information configured in it) and then drop to a command prompt by going to start->run (possibly may have to go to start->all programs->accessories on Vista) and typing in cmd (may have to run cmd in admin mode on vista which requires right clicking cmd and clicking run as admin) and at the dos prompt type in arp -a and hit enter. The arp command should show the mac addresses and associated IP's of all interfaces connected to your computer including the IP and Mac of the AT&T DSL modem. Write down all IP addresses listed that are in the 192.x.x.x range (i.e. such as 192.168.1.1 192.168.2.1 192.168.100.254 etc) and then launch your web browser and type in http://IPaddress (i.e. if one of the addresses was 192.168.1.1 type in http://192.168.1.1 and hit enter). Repeat these steps if nothing happens with any other 192.x.x.x addresses that were listed when you had typed in the arp -a command. One of the addresses should bring up the AT&T DSL modems logon screen. At that point you would need to know the default admin/password to access it or else you would need to use the pin hole reset on the AT&T modem to wipe out the password and to place the modem in bridge mode.

Of course, since I am not familiar with the model of modems AT&T uses I will not recommend using the pin hole reset as I do not know if AT&T has flashed their modems with firmware and hard coded your user info in it, Its possible that AT&T may not allow you to hard reset their modems I am not familiar with their policies. The modems on the IS{ I work for are usually not hard coded with the DSL userid and password and come with a config CD that users often run. When you run the CD, it actually sets up PPPoE auth on our modems and sets them for "non bridged mode" i.e. a mode that may cause issues with 3rd party routers. If AT&T similarly sends out a cd to configure their modems, I would recommend not using it and configuring your router first to see if it will work without setting up the modem as the modem may default to bridge mode as our modems do.

In any case, you have potentially two or three options. If At&T offers DSL modems that default to bridge mode, opt for one of them as it will make life easier. If the AT&T modems have a reset button, ask AT&T if it is okay to reset the modem with the pin hole reset, if they have no issues with it reset it then setup your router and do not bother changing the IP and see if it works this way (likely it will as the pin hole reset on the DSL modem will likely wipe the settings thus placing the modem in bridge mode therefore the DHCP server will not be active on the dsl modem and will therefore not conflict with your routers dhcp).

The third option is to change the dhcp ranges of your router and dsl modem, There are other options as well but these are the two best options.