I am pretty sure that is welded wire fencing, not netting. I saw some one using it for cut flowers to be sold. First layer is placed about 18 " off the ground, and then additional layers added as the plants grow through them. You better have some heavy duty stakes or you could lose them all if it topples.

This is my first year growing dahlias. I planted tubars my neighbour gave me into large pots. They have grown about 1 foot height now, I placed a tomato cage over the plant, instead of staking it...will that do the trick?

Someone else on the forum does that, but puts the tomato cage upside down and then cuts off the wire at the top and bends it to make a stake to keep the upside down tomato cage in place.

I use the following:
staking
tomato cages upside down
green plastic tomato cages with movable connecting pieces (sorry this is not clear)
sheep's netting vertically in rows like in cut flower greenhouses

So far, I like the upside down tomato cages and the sheeps' netting best.

I have 250 plants, so staking takes too much time.

Hope that helps.

Here is a link that might be useful: pictures of tomato cages upside down

why does the cage need to be upside down?

Because of the way the dahlia grows. It is wide at the bottom and narrow at the top where the flowers are blooming.