In WhatsUp Gold, devices are virtual representations of resources (computers/workstations, servers, routers, switches, etc.) that are connected to your computer through a LAN (Local Area Network), a wireless network, or even over the Internet. WhatsUp Gold watches these devices through a network connection. When those network resources are unreachable by WhatsUp Gold, the device is considered down and an action can be configured to fire.
WhatsUp Gold associates Active Monitors with devices on your network. Active monitors query the network services installed on a device and then wait for a response. These monitors query the services running on a network resource, checking to make sure that the FTP server, web server, email server, etc., is up and responding. Active Monitors include DNS, SNMP, Telnet, Ping, TCPIP, and NT Service. If a response is either not received or is not what is expected, the service is considered down. If the query is returned as expected, the service is considered up.
For a more information about service monitors, see Active Monitors Overview.