Andy Chase

Windows/Linux users may find this enlightening (especially if you haven't ever bothered/needed to delve into the finer points of how Darwin interacts with network hardware)

I spent much of yesterday wondering why my Powerbook's ethernet hardware had apparently stopped working some time overnight; no "Built-in Ethernet" in the System Settings "Network" panel, and ifconfig en0 merely indicated that the interface existed, but was inactive. Trying to bring it up manually with ifconfig en0 up did nothing, and trying to force the interface's media type resulted in the error SIOCSIFMEDIA: Input/Output Error.

My ethernet connection under OS X has always "Just Worked", and I was operating under the assumption that ethernet devices behave more or less the same way they do in Windows/Linux, where even if you're not connected to a hub or switch, they will activate and assign themselves a bogus 169.x.x.x IP address. So, I figured I would be subjected to a call to Apple support first thing Monday morning.

Because I don't have wireless at home yet, I have a long CAT-5 cable kicking around the living room, consisting of two shorter cables joined by a coupler. Yesterday evening I noticed that one of the cables had popped out of the coupler... sure enough, once I plugged it back in and my PowerBook saw a hub on the other end, the en0 interface woke right up and grabbed an IP address... apparently OS X doesn't even enable an interface if it knows it's not connected to anything. Which is kind of nice - if you know that that's expected behavior.