How to Monitor Your Network for Suspicious Activity

Every single day, thousands of UK businesses experience unauthorised access attempts, malware infections, and data exfiltration events — many without ever knowing it happened until weeks or months later when the damage has already been done. The average time to detect a network breach in the United Kingdom stands at approximately 212 days according to recent industry research. During that extensive dwell time, attackers can move laterally through your internal systems, harvest user credentials, exfiltrate sensitive commercial and personal data, establish persistent backdoors that survive basic remediation attempts, and position themselves to deploy ransomware at the moment of maximum impact.

Network monitoring is the systematic practice of continuously observing your network traffic patterns, device behaviour, user activity, and system logs to identify anomalies that could indicate malicious activity. It is one of the most effective defensive measures available to UK businesses of any size, yet it remains surprisingly underutilised among small and medium enterprises. This comprehensive guide explains how network monitoring works in practice, what tools and techniques are available at different budget levels, and how to implement an effective monitoring strategy that protects your organisation against both known and emerging threats.

Understanding Network Monitoring Fundamentals

Network monitoring encompasses several related but distinct disciplines that work together to provide comprehensive visibility into what is happening across your entire infrastructure. At its most fundamental level, it involves systematically collecting data from network devices, servers, endpoints, cloud services, and applications, then analysing that data using both automated rules and human expertise to distinguish normal legitimate behaviour from potentially malicious activity.

There are two primary categories of network monitoring that every organisation should understand. Performance monitoring focuses on availability, throughput, and service health — ensuring that devices are online, network links are not congested, applications are responding within acceptable timeframes, and services are meeting their availability targets. Security monitoring focuses specifically on identifying threats — detecting unusual traffic patterns, unauthorised access attempts, malware command-and-control communications, lateral movement within the network, and data exfiltration attempts. Both categories are essential for a well-managed network, but for the purposes of detecting and responding to suspicious activity, this guide focuses primarily on security monitoring.

Recognising Suspicious Network Activity

Before you can effectively monitor for suspicious activity, your team needs to understand what suspicious actually looks like in a network context. The following indicators of compromise are among the most common signals that network monitoring systems are designed to detect and alert upon.

Unusual traffic volumes at unexpected times — such as large data transfers occurring at three o'clock in the morning when no legitimate users should be working — frequently indicate data exfiltration by an attacker or compromised system. Connections to known malicious IP addresses, domains, or command-and-control infrastructure suggest that a device on your network has been compromised and is communicating with attacker-controlled servers. Multiple failed authentication attempts from a single source IP address or against a single account point to a brute-force or credential-stuffing attack in progress. DNS queries to unusual, newly registered, or algorithmically generated domains may indicate malware using DNS tunnelling for covert communication channels. Lateral movement patterns — where one internal device suddenly begins scanning ports, probing services, or establishing connections to many other internal devices it does not normally communicate with — strongly suggest that an attacker has gained initial access and is actively exploring your network to identify valuable targets.

Essential Monitoring Tools and Technologies

Firewall Logging and Analysis

Your network firewall is simultaneously the first line of perimeter defence and one of the richest sources of security telemetry available. Modern next-generation firewalls from vendors such as Fortinet, Sophos, Palo Alto Networks, and WatchGuard generate extremely detailed logs of every connection attempt — both those that were permitted by policy and those that were blocked. By systematically analysing these logs over time, you can identify suspicious patterns such as repeated connection attempts to unusual destination ports, outbound traffic to known suspicious IP ranges or geographies, unexpected protocol usage, and security policy violations.

Configure your firewall to log all traffic comprehensively, not merely blocked traffic. Many organisations make the critical error of only logging denied connections, which means they completely miss the far more dangerous scenario: malicious traffic that was permitted by the firewall because it successfully disguised itself as legitimate business communication. Forward all firewall logs to a centralised logging platform or SIEM for long-term secure storage, correlation, and retrospective analysis.

Intrusion Detection and Prevention Systems

An Intrusion Detection System analyses network traffic flowing through your infrastructure in real time, comparing observed traffic against extensive databases of known attack signatures, protocol anomaly rules, and behavioural heuristics. When the IDS detects a pattern matching known malicious activity, it generates an alert for investigation. An Intrusion Prevention System extends this capability by not only detecting but automatically blocking the identified threat before it reaches its target, providing active rather than passive protection.

Most modern next-generation firewalls include integrated IDS and IPS functionality as a standard feature, but dedicated standalone network IDS appliances and sensors offer significantly deeper analysis capabilities, more extensive signature libraries, and the ability to inspect traffic at wire speed without impacting firewall performance. For organisations with complex network architectures or high-throughput requirements, dedicated IDS sensors deployed at strategic network points provide superior visibility.

Security Information and Event Management

A SIEM platform represents the central nervous system of any mature security monitoring programme. It aggregates logs, alerts, and telemetry from across your entire infrastructure — firewalls, switches, servers, endpoints, cloud services, identity providers, and applications — into a single unified dashboard and analysis engine. The true power of a SIEM lies in its ability to correlate events across multiple disparate sources to identify complex multi-stage threats that no single device or tool would detect in isolation.

For example, a well-configured SIEM might correlate a failed VPN login attempt from an unusual geographic location with a subsequent successful login from the same location just two minutes later, followed immediately by access to sensitive file shares containing financial data. Individually, each of these events might appear benign. Together, the pattern is strongly indicative of a compromised credential being used by an attacker — first testing the credential, then using it to access valuable data. Without cross-source correlation, this attack sequence would go completely unnoticed.

Monitoring Tool	Primary Function	Best For
Firewall Logs	Connection tracking and perimeter policy enforcement	Network perimeter visibility
IDS / IPS	Real-time signature-based and anomaly threat detection	Known attack patterns and exploits
SIEM Platform	Multi-source log aggregation and event correlation	Complex cross-platform threat detection
NDR (Network Detection and Response)	AI-driven behavioural traffic analysis	Unknown threats, zero-day anomalies
EDR (Endpoint Detection and Response)	Endpoint process and behaviour monitoring	Endpoint-level threats and forensics

Implementing Monitoring: A Phased Practical Approach

Phase 1: Establish Your Network Baseline

Before you can reliably identify anomalies, you must first thoroughly understand what normal looks like for your specific network. Spend a minimum of two to four weeks collecting comprehensive baseline data about your network traffic patterns, user behaviours, and system communications. Document which services and applications are in regular use, what times of day traffic naturally peaks, which external destinations are routinely accessed by your business applications and users, how much data typically flows in and out across different time periods, and which internal communication patterns between servers and services are expected and normal.

Phase 2: Deploy Core Monitoring Infrastructure

With your baseline established, enable comprehensive logging on your firewall with all traffic categories captured and forwarded. Configure your endpoint detection and response platform to report telemetry to a central management console. Implement a SIEM or managed log analysis solution appropriate to your organisation's size and budget. For UK SMEs, cloud-based SIEM solutions such as Microsoft Sentinel, Huntress Managed SIEM, or Arctic Wolf offer enterprise-grade detection and correlation capabilities without the complexity, staffing requirements, and capital expenditure of on-premises deployment.

Phase 2b: Network Flow Analysis and DNS Monitoring

In addition to device-level logging and endpoint telemetry, two supplementary monitoring data sources provide exceptional value for detecting suspicious activity: network flow analysis and DNS monitoring. Network flow data — captured via NetFlow, sFlow, or IPFIX protocols from your routers and switches — provides a bird's-eye view of all communication patterns across your network. Flow data records which devices communicated with which, on which ports, for how long, and how much data was transferred, without capturing the actual content of the communications. This makes flow analysis both privacy-friendly and extraordinarily useful for detecting lateral movement, data exfiltration, and unusual communication patterns that would be invisible at the individual device level.

DNS monitoring captures and analyses all domain name resolution requests originating from your network. Since virtually all internet communication begins with a DNS lookup, monitoring DNS traffic provides visibility into every external destination your users and devices are attempting to reach. DNS monitoring can detect connections to known malicious domains, identify DNS tunnelling activity used by sophisticated malware for covert communication, and flag queries to newly registered or algorithmically generated domains that are commonly associated with botnet command-and-control infrastructure. Several UK-focused DNS security services, including the NCSC's own Protective DNS service available to qualifying UK organisations, provide automatic blocking of known malicious domains alongside comprehensive logging and alerting capabilities.

Phase 3: Define and Tune Alert Rules

Configure detection alerts for the suspicious activities most relevant to your business risk profile and threat landscape. Start with high-confidence, low-noise detection rules — alerts that are very likely to indicate genuine threats rather than generating excessive false positives that overwhelm your team. Priority examples include connections to known malicious domains or IP addresses, multiple failed authentication attempts against privileged accounts, unusually large data transfers outside normal business hours, and new administrative account creation outside of approved change windows. Gradually refine, tune, and expand your detection rules as your monitoring programme matures and your team develops experience with the alert volumes.

Phase 4: Establish Incident Response Procedures

Monitoring is only valuable if your organisation has the processes and capability to respond effectively to what it reveals. Define clear, documented incident response procedures that specify exactly who is notified when different severity alerts fire, what initial investigation and triage steps should be taken for each alert category, at what point and through what escalation path an alert becomes a formal incident, and how incidents are documented, contained, remediated, and reviewed. Without well-practised response procedures, even the most sophisticated monitoring system simply generates alerts that nobody acts upon effectively.

Managed Detection and Response for UK SMEs

For many small and medium-sized UK businesses, building and operating an in-house security monitoring capability is neither practical nor cost-effective. The skills required to operate a SIEM, tune detection rules, investigate alerts, and respond to incidents are specialised and in high demand — security analysts in the UK command salaries of £45,000 to £75,000, and a 24/7 monitoring operation requires a minimum of five to six full-time analysts to maintain continuous coverage across shifts, holidays, and absences.

Managed Detection and Response services provide an alternative path that delivers enterprise-grade monitoring capability at a fraction of the cost of building in-house. An MDR provider deploys monitoring technology across your environment, staffs a security operations centre with experienced analysts who monitor your systems around the clock, investigates alerts on your behalf, and either responds directly to confirmed threats or escalates them to your team with detailed remediation guidance.

When evaluating MDR providers for your UK business, assess their detection capabilities across your specific technology stack, their average time to detect and respond to genuine threats, the depth and quality of their investigation and analysis processes, their UK presence and understanding of UK regulatory requirements including GDPR and Cyber Essentials, and their ability to provide actionable intelligence rather than simply forwarding raw alerts. The best MDR services act as a true extension of your team, providing not just monitoring but continuous improvement recommendations that strengthen your security posture over time.

UK-focused MDR providers and platforms include Arctic Wolf, Huntress, Sophos MDR, and Microsoft's own Defender for Business with managed response capabilities. Pricing typically ranges from £15 to £40 per endpoint per month, which for a 50-endpoint organisation represents an annual investment of £9,000 to £24,000 — significantly less than the cost of a single security analyst and delivering continuous coverage that no single hire could provide alone.

Common Monitoring Mistakes to Avoid

Many organisations invest in deploying monitoring tools but then make critical errors that undermine their effectiveness. A SIEM that has not been properly tuned in six months will generate such an overwhelming volume of false positive alerts that genuine threat indicators are lost in the noise — a condition known as alert fatigue. Equally, a firewall generating thousands of detailed log entries per day provides zero security value if nobody is reviewing, analysing, or acting upon those logs in a timely manner.

Log retention is another area where organisations frequently fall short. Many UK compliance frameworks and best-practice standards recommend retaining security logs for a minimum of twelve months, with some regulated sectors requiring longer retention periods. If your logs are only retained for 30 days — a common default setting on many devices and platforms — you will be unable to investigate incidents that are discovered after that window closes. Given that the average breach detection time is 212 days, 30-day log retention is clearly inadequate for most organisations. Configure your SIEM or log management platform to retain relevant security logs for at least 12 months, and ensure that stored logs are protected from tampering by using write-once storage or log integrity verification mechanisms.

Another extremely common error is monitoring only the network perimeter whilst completely ignoring internal east-west traffic between devices within your network. Modern attackers frequently gain their initial access through phishing emails, compromised user credentials, or supply chain attacks that bypass perimeter defences entirely. If your monitoring programme only watches north-south traffic entering and leaving your network, you will entirely miss the lateral movement, privilege escalation, and internal reconnaissance that occurs once an attacker has established their initial foothold inside your environment.