If you work in security long enough, you learn that the best way to understand your network is to try breaking it, with permission, before someone else does. That is all network penetration testing is, and in this post I’ll walk you through how it works, step by step, in plain language.
What is network penetration testing?
A network penetration test (pentest) is a rehearsal of a real attack. With written permission, someone plays the attacker against your servers, firewalls, switches, Wi-Fi and VPNs, and tries to get in, move around and reach the data that matters. What you get back is proof of what is actually exploitable, not a list of maybes.
It’s easy to confuse this with a vulnerability scan, so let me separate them. A scan is automated and broad: it tells you what might be wrong. A pentest is led by a person: they chain weaknesses together, check what really works, and show you what it means for the business.
Why it matters:
- Real risk, not theoretical risk. A medium-severity flaw on an isolated test box matters far less than the same flaw on a domain controller.
- Tests your defences too. A good test shows whether your monitoring and incident response noticed the attack.
- Compliance. Standards such as PCI DSS, ISO 27001 and SOC 2 expect regular testing.
Types of network penetration tests
The type you pick depends on which attacker you want to model.
| Type | Attacker modelled | Starting point | Typical goal |
|---|---|---|---|
| External | Anonymous internet attacker | Public IPs and domains only | Breach the perimeter |
| Internal | Malicious insider or compromised laptop | A network port or low-privilege account | Escalate to domain admin |
| Wireless | Attacker in the car park | Wi-Fi range of the office | Join the corporate network |
| Segmentation | Attacker already inside one zone | A foothold in a low-trust VLAN | Reach restricted zones such as payment systems |
| Cloud network | Attacker targeting cloud workloads | Internet or a stolen credential | Reach VMs, storage and management planes |
Testing can also be black box (no prior knowledge), grey box (some credentials and diagrams) or white box (full access to documentation). Grey box usually gives the best value for time spent.
The methodology: seven phases
Most professional tests follow a structure close to PTES or NIST SP 800-115.
- Scoping and rules of engagement. Agree targets, dates, testing hours, what is off limits and who to call if something breaks. Get this in writing.
- Reconnaissance. Collect information about the target: DNS records, IP ranges, exposed services, employee emails, leaked data. Passive first, then active.
- Scanning and enumeration. Find live hosts, open ports, service versions and shares. This is where most of the picture is built.
- Vulnerability analysis. Match what you found against known weaknesses and misconfigurations, then decide what is worth testing by hand.
- Exploitation. Safely prove that a weakness can be used: weak credentials, unpatched services, misconfigured access, poor segmentation.
- Post-exploitation. Show impact: privilege escalation, lateral movement, access to sensitive data. Stop where the rules of engagement say to stop.
- Reporting and retest. Document findings with evidence and fixes, then verify the fixes work.
Tools of the trade
Tools support the tester; they do not replace judgement. These are the standard, widely documented ones.
| Phase | Tool | What it does |
|---|---|---|
| Reconnaissance | Whois, dig, theHarvester | Domain, DNS and public information gathering |
| Scanning | Nmap | Host discovery, port scanning, service and version detection |
| Scanning | Masscan | Very fast port scanning across large ranges |
| Vulnerability analysis | Nessus, OpenVAS | Automated vulnerability scanning |
| Exploitation | Metasploit Framework | Exploit development and validation platform |
| Traffic analysis | Wireshark, tcpdump | Packet capture and protocol inspection |
| Credentials | Hydra, Hashcat | Password auditing and hash cracking |
| Active Directory | BloodHound, Impacket | Mapping attack paths in Windows domains |
| Wireless | Aircrack-ng | Wi-Fi security assessment |
Most people run these from Kali Linux or Parrot OS, which bundle them. If you’re learning, my suggestion is to build a small lab with VirtualBox and a deliberately vulnerable machine like Metasploitable, or use the free rooms on TryHackMe and Hack The Box. Making mistakes there costs you nothing.
Hands-on: a first scan in your own lab
Run this only against machines you own, such as a home lab or a deliberately vulnerable VM on an isolated network. Here 192.168.56.0/24 stands in for your lab subnet.
Step 1: find live hosts.
nmap -sn 192.168.56.0/24The -sn flag does a ping sweep and skips port scanning, so it is quick and quiet.
Step 2: scan the ports and identify services on one host.
nmap -sV -sC -p- -oA lab_scan 192.168.56.101-sVdetects service versions.-sCruns Nmap’s default safe scripts.-p-scans all 65,535 TCP ports, not just the common ones.-oAsaves results in all formats for your notes and report.
Step 3: read the results like a tester. For each open port, ask three questions: Should this be exposed at all? Is the version old enough to have known flaws? Does it allow default or weak authentication? An old SMB or FTP service, an exposed database port or a management interface open to the whole network are classic findings.
Step 4: note it down. Record the command, the date, the output and why the finding matters. Good notes become a good report.
Reporting, remediation and what defenders should see
The report is the real deliverable. A strong one has an executive summary a manager can read in two minutes, then technical findings that each include a severity rating (CVSS or similar), evidence such as screenshots and command output, business impact, and clear fix steps.
Common network findings, and the usual fix:
| Finding | Usual remediation |
|---|---|
| Unpatched services | Patch management with a defined SLA by severity |
| Default or weak credentials | Strong unique passwords, MFA, a password manager or vault |
| Flat network with no segmentation | VLANs, firewall rules, least-privilege network access |
| Exposed management interfaces (RDP, SSH, admin consoles) | Restrict to VPN or jump host, no direct internet exposure |
| Legacy protocols (SMBv1, LLMNR, NetBIOS) | Disable what is not needed |
| Missing logging and alerting | Centralise logs in a SIEM and tune detections |
I like to think of a pentest as a free fire drill for the blue team. Scanning, brute-force attempts and lateral movement all leave traces: bursts of connection attempts across many ports, repeated failed logons, odd SMB or RDP connections between workstations. If your SIEM, such as Microsoft Sentinel, stayed quiet while the tester was at work, that is a finding in itself, and the best argument for improving your detections.
Always retest after fixing. A finding is closed only when it is verified closed.
Rules, ethics and key takeaways
A word on ethics, because it matters more than any tool. Penetration testing is legal only with explicit, written permission from the owner of the systems. Testing without it can break computer misuse laws, including India’s IT Act, 2000, even when your intentions are good. Stay inside the agreed scope, protect any data you touch, report critical issues straight away, and never test shared infrastructure, such as a cloud provider’s own platform, beyond what that provider’s policy allows.
Takeaways
- A pentest proves exploitability; a vulnerability scan only lists possibilities.
- Follow a method: scope, recon, scan, analyse, exploit, post-exploit, report.
- Learn Nmap and Wireshark first; they teach you how networks really behave.
- Practise only in a lab you own or on legal training platforms.
- The goal is a safer network, so fix, retest and improve detection.
If you’re just starting out, my advice is simple: build a small lab this weekend, run the Nmap steps above and write up what you find. That habit of testing, noting and explaining is what turns a learner into a good security professional. If you try it, tell me how it went in the comments.