In information systems, data is collected, stored and processed, as well as information, knowledge and digital products are protected. The best solution for these requirements is an ISP network monitoring system. ISP networks are distributed and heterogeneous, which makes monitoring crucial for their health. For continuous monitoring, some monitoring systems use open-source software tools. It contributes to the monitoring of systems, applications, services, networks, and infrastructure. We can use these tools to detect network or server problems, share network services or devices among specific groups, generate alerts by simplifying the monitoring process, and save logs for creating reports. Our paper discusses how an ISP can monitor its network using a system such as Nagios. This open-source system provides real-time monitoring and alerts network administrators to anomalies as soon as they occur. We will also present the advantages that Nagios offers to an Internet Service Provider (ISP) for monitoring network equipment and services. We focus on using Nagios as a tool for automatic detection of anomalies occurring in the network topology as well as notification to the Network Monitoring Center (NMC) via email or web interface access.