How to stop auto clustering

Hi guys,

I'm new to ES and found it is a really useful framework.
We (in two machines with same local network) setup two ES instances in two machines.
We found that these two instances have auto clustered (I assume so).
In my machine I could see other ES instance which running on different machine.

Is this feature called clustering ??

I want to avoid this and I just don't want to cluster my ES instance with others.

can anyone explain how to do this...

Thanks,
Dilaa