I'm not the author but anything like this is likely done with a wildcard DNS entry. So basically *.at.hn points to a server app. This app can then see which dom

I'm not the author but anything like this is likely done with a wildcard DNS entry. So basically *.at.hn points to a server app. This app can then see which domain was requested and do whatever it wants with this string, like treat it as a username.