How should I implement sub-windows in my OpenGL viewport?

How should I implement sub-windows in my OpenGL viewport? Inside my viewport, I want to reserve some space on the left for labels, and some space around the edges as a border. I've got all the coordinates figured out and everything is displaying properly. My problem is clipping the things in one subwindow that are spilling over into the others. I can't seem to figure out what the OpenGL 3.3, core context way of doing things is. Is it to

use per-vertex clipping?
a scissor test?
a stencil test?
associate a framebuffer with different parts of my window?

Which commands should I be looking at?

Solution

Before I spend time writing a full answer, I would like you to confirm that this is what you were describing in your original question:

*---------------------------------------*
| ------------------------------------- |
| |   |                               | |
| |   |                               | |
| |   |                               | |
|C| A |              B                |C|
| |   |                               | |
| |   |                               | |
| |___|_______________________________| |
*---------------------------------------*

A = Labels
B = Main Window
C = Border