Network Penetration Testing: What It Is and How It Works

## What Is Network Penetration Testing? Network penetration testing is an authorised, simulated attack against your network infrastructure — servers, firewalls, VPNs, domain controllers, workstations, and the services running on them — carried out to prove which weaknesses an attacker could actually exploit. Where an application pentest targets your software, a network pentest targets the environment that software runs in. It answers questions a scanner cannot: could an attacker on the internet get a foothold inside? Once inside, how far could they go, and how quickly could they reach domain admin or your customer data? For a general introduction to the discipline, see [what penetration testing is](/blog/what-is-penetration-testing). ## External vs Internal Network Testing ### External Network Penetration Testing Starts from the internet with no prior access, mimicking an opportunistic or targeted outside attacker. Typical scope: public IP ranges, VPN and remote-access portals, mail and DNS infrastructure, exposed management interfaces, and any forgotten host that discovery turns up. Common findings: exposed RDP or SSH, legacy VPN appliances with known CVEs, default or reused credentials, successful password spraying against SSO or webmail, and misconfigured cloud security groups. ### Internal Network Penetration Testing Assumes a breach has already happened — a phished employee, a rogue contractor, or a compromised laptop — and measures blast radius. Typical scope: internal subnets, Active Directory, file shares, database servers, hypervisors, and segmentation boundaries. Common findings: Kerberoastable service accounts, LLMNR and NBT-NS poisoning leading to credential relay, over-privileged accounts, unconstrained delegation, weak or shared local administrator passwords, and flat networks with no meaningful segmentation. If Active Directory is central to your estate, our [Active Directory security review](/active-directory-security) goes deeper on this ground. Most mature programmes run both, because they answer different questions. ## The Methodology Credible engagements follow PTES and NIST SP 800-115 rather than a tool's default profile: 1. **Scoping and rules of engagement** — targets, timing windows, exclusions, escalation contacts, and evidence handling agreed in writing. 2. **Reconnaissance** — passive intelligence gathering, DNS and certificate enumeration, and identification of the real attack surface, which is almost always larger than the client's list. 3. **Enumeration and vulnerability analysis** — service fingerprinting, configuration review, and correlation of findings, with automated output triaged by hand. 4. **Exploitation** — controlled, evidence-driven exploitation to prove impact rather than assert it. 5. **Post-exploitation and lateral movement** — privilege escalation, credential harvesting, pivoting, and testing whether segmentation actually holds. 6. **Reporting and retesting** — a prioritised report with reproduction steps, then verification once fixes are in. Our testers are OSCP-certified and every finding is manually validated, so you do not pay for scanner noise. ## What a Credible Report Contains - An executive summary written for non-technical decision makers - Each finding with risk rating, affected assets, reproduction steps, and evidence - The full attack narrative — how individually "medium" issues chained into critical impact - Specific, actionable remediation guidance, not vendor boilerplate - A remediation retest and an attestation letter you can share with customers and auditors ## Timeline and Effort | Scope | Active testing | End to end | |---|---|---| | External, up to ~50 hosts | 3–6 tester-days | 1–2 weeks | | Internal / Active Directory | 5–10 tester-days | 2–3 weeks | | Combined external and internal | 8–14 tester-days | 3–4 weeks | Effort, not host count alone, drives price. See the [penetration testing cost guide](/blog/how-much-does-a-penetration-test-cost) for indicative ranges by engagement type. ## Why Compliance Frameworks Ask For It - **SOC 2** — independent testing is the standard evidence for the risk-assessment and monitoring criteria. US SaaS teams preparing for audit should start with [SOC 2 penetration testing](/soc2-penetration-testing). - **PCI DSS** — Requirement 11.4 mandates internal and external testing annually and after significant change, plus segmentation testing for in-scope environments. - **ISO 27001** — supports Annex A technical-vulnerability and network-security control evidence. - **NDPA 2023 (Nigeria)** — demonstrates the technical measures required to protect personal data, and is central to [NRS integration security assessments](/nrs-integration-security-assessment) for certified System Integrators and Access Point Providers. ## How Often Should You Test? Annually at minimum, and additionally after any material change: a new data centre or cloud region, a merger, a domain migration, a new remote-access solution, or a significant segmentation change. Between tests, run a continuous [vulnerability management](/blog/what-is-vulnerability-management) programme — testing proves depth, the programme maintains breadth. ## Frequently Asked Questions **What is network penetration testing?** An authorised simulated attack on your network infrastructure to identify and prove exploitable weaknesses before a real attacker finds them. **What is the difference between internal and external network penetration testing?** External testing starts from the internet with no access and measures whether an outsider can get in. Internal testing assumes a foothold already exists and measures how far an attacker could move. **How long does a network penetration test take?** Typically 3–10 days of active testing, with reporting delivered inside 1–3 weeks depending on scope. **Is a vulnerability scan the same as a network penetration test?** No. A scan lists known issues automatically. A penetration test manually exploits and chains them to prove real business impact. **How often should network penetration testing be done?** At least annually, and after any significant infrastructure, authentication, or architecture change. PCI DSS requires it explicitly. ## Find Out What an Attacker Would See The only way to know whether your perimeter and internal segmentation hold is to have someone competent try them. **Next step:** [View our penetration testing service](/penetration-testing), [run the free security scorecard](/security-scorecard), or [book a 15-minute scoping call](/growth-partner).