Network Monitoring Basics: How to Detect Suspicious Activity

Why You Should Monitor Your Network

Network monitoring is the practice of observing the devices, traffic, and connections on your network to identify problems and security threats. Most people set up their home or office network and never think about it again, but an unmonitored network is an open invitation for unauthorized access, data theft, and malicious activity.

The average home network today connects dozens of devices, including computers, smartphones, tablets, smart TVs, security cameras, voice assistants, and IoT devices. Each of these devices represents a potential entry point for attackers. A compromised smart device can be used to spy on your household, launch attacks against other targets, or serve as a pivot point to access more valuable devices on the same network.

Monitoring does not require expensive enterprise tools or deep networking expertise. Simple, free tools can reveal unauthorized devices, unusual traffic patterns, and connections to suspicious destinations. Developing a basic understanding of your network's normal behavior makes it possible to notice when something is wrong.

Signs of Network Compromise

Several indicators may suggest that your network has been compromised. Unexplained slowdowns in internet speed can indicate that someone is using your bandwidth, whether a neighbor who obtained your Wi-Fi password or malware on a device that is uploading data or participating in a botnet.

Unknown devices appearing on your network are an obvious red flag. If your router's connected devices list shows devices you do not recognize, investigate immediately. This could indicate that someone has gained access to your Wi-Fi network or that a device you forgot about is connecting.

Unexpected outbound connections to unfamiliar IP addresses, especially in countries where you have no contacts or business relationships, may indicate that a device is communicating with a command-and-control server. High volumes of DNS queries, particularly to recently registered or suspicious domains, can signal malware activity.

Unusual traffic at odd hours, when all household members are asleep and devices should be idle, warrants investigation. While some legitimate background activity is normal, sustained high traffic volumes during off-hours are suspicious.

Network Scanning Tools

Fing

Fing is one of the most user-friendly network scanning tools available. The free mobile app scans your local network and displays every connected device, including its IP address, MAC address, device name, and manufacturer. Fing can identify device types, detect open ports, and alert you when new devices join your network.

The Fing app provides a simple way to inventory your network and establish a baseline of known devices. Review the device list regularly and investigate any entries you do not recognize. Fing also includes a speed test feature that you can use alongside our speed test tool to establish baseline network performance for comparison during suspected issues.

Angry IP Scanner

Angry IP Scanner is a free, open-source network scanning tool for Windows, macOS, and Linux. It scans IP address ranges and reports responding hosts along with open ports, hostnames, and MAC addresses. While more technical than Fing, Angry IP Scanner provides detailed information useful for thorough network auditing.

You can save scan results and compare them over time to identify changes in your network. New hosts, newly opened ports, or devices that have changed their network configuration are all worth investigating.

Wireshark

Wireshark is the gold standard for network traffic analysis. This free, open-source packet analyzer captures and displays detailed information about every packet on your network. While Wireshark requires more technical knowledge than Fing or Angry IP Scanner, it provides unmatched visibility into network communications.

With Wireshark, you can identify which devices are communicating with which servers, examine the protocols being used, and detect suspicious traffic patterns. Wireshark's filtering capabilities allow you to focus on specific devices, protocols, or destinations, making it practical even on busy networks.

Checking Connected Devices

Your router's administration interface provides the most authoritative list of connected devices. Access it by navigating to your router's IP address in a web browser, typically 192.168.1.1 or 192.168.0.1. Log in with your administrator credentials and navigate to the connected devices or client list section.

Review this list and identify every device. Compare the number of connected devices against what you expect. If you count twelve devices but only know of ten, the two extras need investigation. Note the MAC addresses of all legitimate devices so you can quickly spot unfamiliar entries in the future.

Some routers allow you to set up MAC address filtering, which restricts network access to a predefined list of approved devices. While MAC addresses can be spoofed by a determined attacker, this adds a layer of defense against casual unauthorized access.

Traffic Analysis Basics

Understanding basic traffic patterns helps you identify anomalies. Normal home network traffic consists primarily of web browsing (HTTP/HTTPS on ports 80 and 443), email, streaming video, and DNS queries (port 53). Traffic to unusual ports, especially those commonly associated with remote access tools, peer-to-peer networks, or known malware communications, should be investigated.

Monitor the volume of traffic generated by each device. A smart thermostat that suddenly starts generating megabytes of outbound traffic is likely compromised. IoT devices should have relatively predictable, low-volume communication patterns, and significant deviations suggest a problem.

DNS monitoring is particularly valuable because almost all network activity begins with a DNS query. Monitoring DNS traffic can reveal connections to known malicious domains, cryptocurrency mining pools, and command-and-control servers. Tools like Pi-hole provide built-in DNS logging and can display which domains each device on your network is querying.

Setting Up Alerts

Proactive alerting turns network monitoring from an occasional manual task into a continuous security control. Many router firmwares and network monitoring tools support email or push notifications for specific events.

Configure alerts for new devices joining the network, which catches unauthorized access quickly. Set up bandwidth threshold alerts to detect unusual data transfers. If your router supports it, enable logging of blocked connection attempts to track scanning and intrusion efforts.

For more advanced monitoring, consider setting up a dedicated monitoring device such as a Raspberry Pi running network monitoring software. Tools like Ntopng provide web-based dashboards showing real-time traffic flows, top talkers, and protocol distribution, making it easy to spot anomalies at a glance.

Establishing Your Baseline

Effective monitoring depends on knowing what normal looks like. Spend a week logging your network activity during different times of day and on different days of the week. Document which devices connect, typical bandwidth usage, and common traffic destinations. This baseline becomes your reference point for identifying future anomalies.

Record your baseline network speed using a speed test tool. Significant drops in performance compared to your baseline may indicate unauthorized bandwidth usage, network congestion from compromised devices, or degradation that warrants investigation.

Secure your network accounts with strong credentials. Use a password generator to create a strong Wi-Fi password and router administration password. A compromised router password gives an attacker complete control over your network, making router credential security a top priority.

Conclusion

Network monitoring is an accessible and valuable security practice that anyone can adopt. By scanning your network regularly with free tools, checking connected devices against your known inventory, understanding basic traffic patterns, and configuring alerts for anomalies, you create an early warning system that can detect intrusions, unauthorized access, and compromised devices before they cause significant harm. Start with a simple device inventory today and build your monitoring capabilities over time.