How to setup a roaming wifi network over ethernet with an Airport Extreme and Airport Express

In the past, all the devices in my home operated over a single wifi network—including four Macs, an AppleTV and several iOS devices. I've long suspected I was probably overtaxing the wifi, and listening to the recent "taming wifi" episode of the Technical Difficulties podcast inspired me to do something about it.

So my project this weekend was running cat-6 cabling throughout the house, allowing me to move my Macs and AppleTV off the wifi and onto a gigabit ethernet.

As part of that project, I wanted to modify my upstairs/downstairs roaming wifi network from one in which an Airport Express extended the network created by an Airport Extreme over wifi, to one in which the Express extended the network over ethernet. That proved a bit trickier than I expected, so I wanted to explain it here for the potential benefit of others.

To clarify what I wanted to do: Previously, the Airport Express (upstairs) connected to the Airport Extreme (downstairs) over wifi (a network called "Hacienda"), and then extended the "Hacienda" wifi network to the upstairs area of my house. Configuring an Express to do this is very easy; you just select "Extend a wireless network".

The problem with such a setup, is that the Express is sharing its wifi bandwidth between two functions—communicating with the upstairs devices over wifi, and then passing that same traffic to the Airport Extreme downstairs, over wifi. Now with cabling everywhere, my plan was to connect the upstairs Express to the ethernet, so that it could extend the wifi network upstairs without having to communicate with the downstairs Extreme over wifi.

Unfortunately, that wasn't as easy as just connecting the Express to the ethernet. In fact, doing just that caused all sorts of problems; all of a sudden my upstairs Macs started popping up those warnings, "This Mac's IP address is already in use by other computer with the same name..."

The solution is to configure the upstairs Express in precisely the same way as the downstairs Extreme—i.e. you "create a wireless network" with the same name ("Hacienda" in my case) and the same password, and set the device to operate in "bridge mode". Doing that, and everything works as expected.

So, wrapping up, here's a diagram of what my network looked like before and after:

If you're looking to do the same thing, I hope this article has helped. And if you have any questions, don't hesitate to write.