LibreNMS is an open-source, auto-discovering network monitoring system based on PHP, MySQL and SNMP. It supports many families of network hardware and operating systems, including Cisco, Linux, FreeBSD, Juniper and HP. It also offers customisable alerting by email, IRC and Slack, an API, bandwidth billing based on port usage, distributed polling, mobile apps and integrations with tools such as Oxidized and SmokePing. You install LibreNMS yourself, on Linux with Apache or Nginx, from Docker images or as a virtual machine.
LibreNMS is a community-based project licensed under the GNU General Public License v3. The project has no first-party hosted service; commercial support is available through partners, including Config Services Ltd (UK). Because you run it on your own infrastructure, monitoring data such as device inventory and network topology stays wherever you host it, for example in a data centre in the EU.