Hex Notation

I recently read an article on IPv6 and the table of contents was in this kind of format:

0x0 - Intro
0x1 - Why the change is needed
...
...
1x0 - The Future of IP
1x1 - etc
1x2 - etc

What is this notation actually called?

Chapters are in base 16, if it wasn't apparent to all you tom-noddy's out there (I'm looking at you, >>1 and >>9)
0x1 - 0xF
1x1 - 1xF
2x1 - 2xF
...
and so on and so forth.