VirtualMetric uses:
WS-Man protocol to connect and monitor Windows servers, therefore, allowing HTTP/HTTPS ports on firewall is enough to start monitoring.
VMware vSphere Web Services to connect and monitor VMware servers. Allowing HTTP/HTTPS ports on firewall is enough to start monitoring.
SSH procotol to connect and monitor Linux servers. Allowing SSH/HTTP/HTTPS ports on firewall is enough to start monitoring.
SNMP protocol to connect and monitor network devices servers. Allowing SNMP/HTTP/HTTPS ports on firewall is enough to start monitoring.