How is it possible for a domain name to exist without a TLD?

For example, I found this website while browsing the Spam folder:

http: //0x505d5a7a/index.php

... as? I don't want this url to not work?

EDIT: decimal and octal representations don't work (or can't):

http: //1348295290/index.php

http: //12027255172/index.php

Bad Request

Your browser sent a request that this server could not understand.

      

        
        
        
      

    

But the dotted hex:

http: //0x50.0x5d.0x5a.0x7a/index.php

And so the dotted octal:

http: //0120.0135.0132.0172/index.php