Network Monitoring falls under the 3rd part of this life-cycle. It allows you to keep an eye on the devices in your network (routers, switches, servers, applications, etc.) so that you can take appropriate actions in the event that it becomes necessary. It also allows for proactive management rather than reacting after the fact.
Several tools are available to help with network monitoring, from open source applications to those that require commercial licenses. Two of such network monitoring tools are Zabbix and Nagios. Nagios sets itself up as the “Industry Standard In IT Infrastructure Monitoring” while Zabbix says it is “the Enterprise-class Monitoring Solution for Everyone”.