Unable to run 5 concurrent tests on a single node in Selenium Grid 2

Case:1  I registered hub on one machine and node on second machine. I tried to execute 5 concurrent tests on the node (5                      instances of firefox), but i am able to run only 4 of them.

Case:2  I registered both hub and node on the same machine and ran 5 tests still i am able to run 4 of them and the  5th Job is                  failing.

Case:3  I registered hub on one machine and registered 5 nodes to the hub on remote machine and executed 5 tests one on each              port. I am able to run 4 of them only and 5th job is breaking in one of the node.

Case:4 I tried to 3 jobs on node and another 3 jobs on second node which are registered for the same hub still able to run 4 tests(2           on each node) .