From OpenNMS

Intro

Before starting OpenNMS, you configure OpenNMS with the relevant address ranges (typically one or more subnets, but the syntax is flexible). Optionally you can configure your local SNMP community strings, with significant flexibility in using different community strings for different address ranges or specific addresses.

Once configured, start OpenNMS, and your network will be automatically discovered. The Discovery daemon sends out ICMP pings to see if something responds on that IP address. If a response is received, a "New Suspect" event is generated and passed into the OpenNMS event system. The Capabilities daemon picks that event up and scans that IP address for available services that OpenNMS knows about. The node and it's services will be added to the OpenNMS database; monitoring and data collection will commence for that node.