This is a bit geeky, and when I first saw the post from 37 Signals Announcing Pow 0.4.0 with xip.io support I really didn’t get it. However, going to the xip.io domain was a bit more revealing.
10.0.0.1.xip.io resolves to 10.0.0.1
www.10.0.0.1.xip.io resolves to 10.0.0.1
mysite.10.0.0.1.xip.io resolves to 10.0.0.1
foo.bar.10.0.0.1.xip.io resolves to 10.0.0.1
If you’re a developer doing web development you will (hopefully) see just how cool this is.