2 routers Motorola SBG900 + airport extreme (AE)

Welcome to the discussions, rick_lv!

+The SBG900 has been my router for a couple of years. Now I'd like to set the AE as my main router and take advantage of the n standard.+

Your SBG900 appears to be a gateway...a combination modem and router...that is most likely designed to function as a complete unit. In order for the AirPort Extreme to be your "main" router, you would need to connect it directly to a modem.

Whether or not the SBG900 can function only as a modem is open to question. Even if it were possible, it would require a number of configuration changes. Sorry, I can't help on that front, and your ISP probably won't be willing to help much either.

It would be far easier to allow the SBG900 to handle DHCP and NAT functions, as it is doing now, and simply connect the AirPort Extreme as a "bridge". When the AirPort Extreme is configured this way, it does not handle DHCP or NAT functions.

+It's my understanding that the DHCP cannot be turned off on the AE, is it correct?+

No, this is not correct. When the AirPort Extreme (or any other router for that matter) is configured as a bridge, DHCP is turned off.

You will be able to setup a dual band 5 GHz and 2.4 GHz wireless network with the AirPort Extreme when it is configured as a bridge. Faster "n" devices will connect to the 5 GHz band and slower devices will connect to the 2.4 GHz band.

But, you will not be able to setup a separate Guest network with this type of configuration. The Guest network feature can only be enabled if the AirPort Extreme is connected directly to a modem, not another router.

Please post back if you feel that using the AirPort Extreme as a bridge would work for you and I'll provide a step by step reply to configure it that way.

Happy to help. Hope you have a couple of spare ethernet cables there as one will be needed to connect the AirPort Extreme (AEBS) and your SBG900 together and the other will be needed to help with configuration of the AEBS. If you don't have two cables handy, use one for the configuration and then move it to connect the AEBS and SBG900 together.

Connect an ethernet cable from one of the LAN <-> ports on the SBG to the WAN port (circle icon) on the AEBS. Connect another cable from one of the LAN ports on the AEBS to your computer.

Open AirPort Utility, click Manual Setup. If the AEBS does not appear, hold in the reset button on the AEBS for 10 seconds, let it restart and try again. You may need to restart your computer as well.

Open AirPort Utility, click Manual setup as above

Click the Base Station tab below the row of icons to name your device, assign a device password, set time zone, etc.

Click the Wireless tab under the icons
Wireless Mode...Create a wireless network
Wireless Network Name....your choice
Check mark next to Allow this network to be extended if you plan to add other Apple routers at a later time to extend the wireless coverage.
Radio Mode...Automatic would be a good choice. If you need more choices, you can hold down the option key on your computer as you click on the selection box.
Wireless Security....WPA/WPA2 Personal
Wireless Password.....your choice

Click the Internet icon in the row of icons above
At the bottom of the page, look for the Connection Sharing setting.
Set Connection Sharing to "Off (Bridge Mode)"

Click Update at the lower right of the window and the AEBS should restart and give you a green light in 20-25 seconds.

You can leave the wireless turned on on the SBG if you want another wireless network operating (maybe the kids could use this network), or you can turn it off if you don't need it. Your choice.

Please post back on your progress.

All looks good. I see you have set it up so the 5 GHz network has a separate name. That's useful if you want to "force" your computer to connect to that network.

Under Radio Mode, I use the "802.11n only 5 GHz - 802.11b/g/n" mode because I don't have any "a" devices, but Automatic should work as well.

Does everything appear to be working there?

5 GHz signals are absorbed by walls and any obstructions much more quickly than 2.4 GHz signals, so you may find that the 5 GHz signal does not work as well if you are some distance from the AEBS.