Mesos webinterface showing agents unreachable 2

[Marc Roos]

In my test setup I have 3 activated agents and 2 unreachable. Although 
tasks are being deployed among all 3. I take it unreachable should be 0. 
How should I troubleshoot this? Could these 2 be some old hosts that I 
maybe installed in the past?

Agents
Activated         3
Deactivated       0
Unreachable       2