How communication between ranks works?

Example of hipo cluster : 

"Node 1" -> 4 cores so [0,4[ rank

"Node 2" -> 4 cores so [4,8[ rank

So rank = 0 would be first core in Node 1

In a situation like Core #5 wants to send a message to Core #6

which are both in "Node 2"

What happens?

I hope Core 5 doesn't sends its message to Rank #0 in "Node 1" that sends it back to Core #6.

So how does it works?