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

Question

For instance, I found this website while going through my Spam folder:

http://0x505d5a7a/index.php

... how? I mean shouldn't this URL not even work?

EDIT: The decimal and octal representations do not (or cannot) work:

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 does:

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

And so does the dotted octal:

http://0120.0135.0132.0172/index.php

Answer 1

This is a hexadecimal noted IP address so this is a valid url.

My browser directly shows me the IP 80.93.90.122