Network Intrusion Detection

Network Intrusion Detection System (NIDS) is either a piece of software or a dedicated appliance that inspects network traffic and raises alerts when suspicious or malicious behavior is detected. These alerts are passed on to security analysts for investigation.

Depending on how it is deployed, a NIDS can be positioned in different ways:

• Inline – When the detection system is placed directly in the traffic path, all packets must pass through it. In this configuration, it acts as an Intrusion Prevention System (IPS), allowing it to actively block or reset malicious connections. The downside is that if the device fails, it can disrupt or block all traffic.
• Network Tap – The system is connected through a dedicated hardware tap that makes a copy of network traffic. This ensures monitoring without interfering with the live traffic flow.
• Passive/SPAN Port – The NIDS is linked to a switch’s SPAN (mirror) port, which sends a copy of traffic to the detection system. This allows full visibility but does not affect normal traffic flow.

In most cases, NIDS solutions are focused on generating alerts for later investigation. Only when deployed inline do they also prevent threats (at which point they are considered IPS).

Popular Network Intrusion Detection Tools

• Snort – One of the most widely used open-source intrusion detection systems. It supports a large library of community-driven rules and can be deployed quickly for immediate threat detection. Snort Website
• Suricata – Another open-source solution that operates at the application layer, providing deep packet inspection and enhanced visibility compared to traditional packet-based systems. Suricata Website
• Zeek (formerly Bro) – A powerful monitoring framework that goes beyond traditional IDS by also providing network analysis and traffic logging capabilities. Zeek Website

Network Intrusion Prevention

A Network Intrusion Prevention System (NIPS) builds on the capabilities of NIDS but goes a step further by taking automatic defensive actions when threats are detected. Instead of just alerting analysts, it can block or contain malicious activity in real time.

For example, if a compromised machine within the network starts scanning other hosts, a NIPS can be configured to instantly drop its connections, block future traffic from that source, and notify analysts.

NIPS Solutions

Most modern tools like Snort, Suricata, and Zeek can operate in both IDS and IPS modes, depending on how they are configured and deployed.

Firewalls

firewall acts as a security boundary, regulating the flow of network traffic between trusted and untrusted zones. For instance, a firewall placed between a company’s internal network and the internet will only allow approved traffic such as HTTP, HTTPS, or DNS, while blocking everything else. This prevents attackers from freely probing or exploiting internal systems.