Find the two closest values in a list (bigger and smaller)

Hi,

I have a list with random ordered numbers (each number in a cell of a row):
1, 2, 6, 8, 11, 12, 16...

On the other hand I get a value (x). I want a formula that:
- If x is in the list, returns x
- If x is not in the list, returns the next higher value. For example, if x=8, returns 11

I want also a variation of the same formula, that returns the next lower value. For example, if x=8, returns 6.

Note that x will never be smaller than the first number on the list or higher than the highest number in the list.

Can you help me with that?