My friend Paul tweeted about a new URL shortener like bit.ly and is.gd but with one major difference.  The domain for this shortener is only 2 letters "t-o" and they're not separated by a dot.  The link to the shortener was posted as http://to./ which appears to be an invalid link since it has no top level domain (com, net, org etc.) but low and behold, it worked.

How it works

So, how is it this seemingly invalid URL actually works?  I wasn't sure either so I did some poking around my DNS log.  The log usually contains queries to domains like www.facebook.com in the form:

www.facebook.com: type A, class IN

to which the DNS server responds:

www.facebook.com: type A, class IN, addr 66.220.146.18

at which point my browser can connect to facebook's web servers at 66.220.146.18.

Now here are the log entries for http://to./

to: type A, class IN
to: type A, class IN, addr 216.74.32.107

Which means the TLD (top level domain) is not a dot nor slash but "to".  There's no 2nd level domain, nor hostname and the TLD resolves to the IP 216.74.32.107.  The fact that a web page appears means that there's a web server running at that address.

Breaking standards?

While this may be breaking convention, I'm not sure it's actually breaking any sandards.  Apache defines an FQDN (fully qualified domain name) as:

The unique name of a network entity, consisting of a hostname and a domain name that can resolve to an IP address.

So "to." is not an FQDN according to Apache but it does resolve to an IP address and it fits Wikipedia's description of FQDN which does not have constraints on the number of names.

Other TLD as FQDN

This makes me wonder if other TLD will ever use their TLD as an FQDN.  I imagine there'd be quite a demand for http://com. http://net. and others but at the time none of them that I tested currently resolve to an IP address.