Does elasticsearch gets clustered by default?

I have two elastic search instances in two different machines in a LAN. I have also installed plugin-head in the two machines. Each of the plugins shows that the two elasticsearch instances are in cluster, one as master and one as worker. I have not changed the elastic search yml. Can you please confirm whether such installations creates a cluster by default?

Prior to ES 2.0, multicast discovery was enabled by default so any ES nodes in the same network and with the same cluster name would form a cluster. This practice isn't recommended anymore, i.e. you should use unicast discovery and explicitly set which nodes should be part of any cluster.

So, if I want to disable this default clustering, what should I do? Shall I upgrade to Elasticsearch 2.1.1 or there is some parameter in the elasticsearch.yml that will disable the default clustering?

You can disable multicast. false

And change your cluster name: mycluster

Also you can bind only to localhost: