We just explored network discovery—it scanned a network range. Zabbix also supports a feature that goes the other way around, where Zabbix agents can chime in and Zabbix server can automatically start monitoring them. This is called active agent autoregistration.
Whenever a Zabbix agent connects to the Zabbix server, the server compares the incoming agent hostname with the existing hosts. If a host with the same name exists, it proceeds with the normal active item monitoring sequence. This includes both enabled and disabled hosts. If the host does not exist, the autoregistration sequence kicks in, that is, an event is generated.
The fact that an event is generated every time an unknown agent connects to the Zabbix server is important. If you do not use active items or autoregistration, switch off active checks on the agent side. Otherwise, each such check results in a network connection, log entry on the agent and server side, and an event in the Zabbix database...