How do I use a 2nd Airport Extreme plugged into an Ethernet Wall Plug to join an existing wireless network to give me a better signal upstairs?

I assume that the new AirPort Extreme is still in its original factory default settings.

Before you connect the AirPort Extreme, please connect a laptop to the Ethernet cable that will be used for the connection and turn off the wireless on the laptop. Make sure that you can can connect to the Internet before you begin to configure the AirPort Extreme.

Disconnect the Ethernet cable from the laptop and connect it to the WAN "O" port on the upstairs AirPort Extreme and power it up.

Open Macintosh HD > Applications > Utilities > AirPort Utility 6.1

Look in the upper left hand corner of the screen for a tab or box that reads "Other AirPort Base Stations (1)"

Click on that box,then click on the name AirPort Extreme xxxxxx

Wait a moment while AirPort Utility analyzes the network and then displays a message that the AirPort Extreme will be configured to extend your existing wireless network.

Enter a Base Station (device) name for the AirPort Extreme (and a device password if prompted), then click Next.

Wait another minute or two until AirPort Utility notifies you that the AirPort Extreme has been Set Up.

That's it.

First you will need a computer connected via wireless or an ethernet cable running the Airport Utility so when you plug in the second Extreme, it will be recognized - you may need an ethernet switch to do this. Next connect the Extreme to the ethernet using the WAN port and power it up - it will take a while but you should see it on the Airport Utility below the first Extreme - now highlight the new Extreme and click "Continue" - this will start the automatic setup procedure - you will be asked a series of questions and be able to name the Extreme - be sure to set it up in "Bridge Mode" and use the same Wireless network name and type of security and password as your base station - and when asked, choose "Create a wireless network" because this is a wired extender - I know this sounds wrong but I made this mistake when I first did this - and I have only done this once and it was a while ago so I hope this helps.

The instructions provided by CRMDVM assume that you are using AirPort Utility 5.x.

The information in your post indicated that you were using Lion 10.7.4, so my instructions track with Lion and AirPort Utility 6.1.

Sorry Bob - as usual you are right - could you please tell me how many wireless guest clients an Extreme can handle? Thanks

Sure, the AirPort Extreme can handle a total of 50 wireless clients between both the main and guest networks.

So, for example you could have 40 on the main network and 10 on the guest network.

Thanks - since we are on the subject, do you know if Apple has any plans to allow the guest network to be extended?

I have no idea what Apple's plans might be, but I also don't see how this could be possible, since a device that extends the network will always be in Bridge Mode...and the guest network cannot be enabled in Bridge Mode.

I believe that this is known as a Catch 22.

I understand - it is frustrating - the management at the museum wants an inexpensive guest network to cover 30,000 sq ft - I can do it with 2 Extremes with another T1 IP address and different subnet but I don't think it is worth the investment for only 50 wireless clients - do you have any suggestions?

The simplest idea would be to set up another AirPort Extreme, or new AirPort Express network on a different subnet. Users on the "main" subnet would normally not be able to "see" users on the "guest" subnet and vice versa. Both subnets would share the same Internet connection and bandwidth.

If the IT security guys came in, it will probably not pass inspection, security wise. Not sure how secure things need to be.

The downsides are that this will split the available bandwidth on the two subnets and create a Double NAT situation on the "guest" network. That may not be an issue if the network will only be used by folks looking for a simple Internet connection from their mobile devices and laptops.

Sorry I took so long to get back to you but I was checking with the T1 provider and their router has a second ethernet port that can provide a free second IP address and subnet that will be totally independent from the current network so if I use an Extreme as a base station can I hook up an Express as a wireless extender and will I still get 50 wireless clients? The bandwidth is provided on demand so there is never a question of too many users on either connection. The speed is constant.

can I hook up an Express as a wireless extender and will I still get 50 wireless clients?

Yes, if you use the "new" AirPort Express which will handle 50 wireless clients.

.....under the Wireless tab the Network Mode is set at "Extend a Wireless Network". The other choice is "Create a Wireless Network". Is "Extend a Wireless Network" correct?

This tells me that the 2nd AirPort Extreme is connecting using wireless to the first AirPort and not Ethernet. The setting should read "Create a wireless network" when the device connects using Ethernet.

Did you check the Ethernet cable that connects to the 2nd AirPort Extreme first with a laptop to verify that that it is working correctly?

There should be a solid line....not a dotted line.....between the two AirPort Extremes when you first open AirPort Utility and look at the network layout.