# What is network penetration testing?

> Network penetration testing explained: internal vs external tests, what testers look for, what you get at the end, and how often to test.

Canonical: https://cybersecuritylondon.com/what-is-network-penetration-testing


By Cyber Security London, published 11 October 2026.

**In short:** Network penetration testing is an authorised, simulated attack on your network by a skilled tester. An external test looks at what anyone on the internet can reach; an internal test assumes an attacker is already inside. You get proof of what could be reached and a prioritised list of fixes.

## Internal vs external network testing

|  | External test | Internal test |
| --- | --- | --- |
| Starting point | The public internet | Inside your network, like a compromised laptop or a visitor's device |
| Looks at | Firewalls, VPNs, mail servers and anything internet-facing | Servers, shared drives, user accounts and how far an attacker could move |
| Answers | Can someone break in from outside? | If someone gets in, how much damage could they do? |

## What do testers look for?

- Missing security updates on servers and network devices.
- Weak, default or reused passwords.
- Services exposed to the internet that shouldn't be.
- Poor separation between parts of the network.
- Paths from an ordinary user account to administrator access.

## What do you get at the end?

A short summary for leaders, a technical report with evidence for each finding, and a prioritised fix list. A retest afterwards confirms the fixes worked.

## How often should you test your network?

At least once a year, and after significant changes such as a new office, a new firewall, a cloud migration or merging networks. Regular vulnerability scanning in between keeps on top of new weaknesses.

## Frequently asked questions

### Is network penetration testing disruptive?

It is planned to avoid disruption: testing windows and limits are agreed in advance, and testers contact you immediately if anything unexpected happens.

### How long does a network penetration test take?

Usually a few days of testing for a small or medium network, followed by the report. The scope sets the timeline.

### Do we need both internal and external testing?

Most businesses benefit from both. External testing shows how hard it is to get in; internal testing shows what an attacker could do once inside, which is how most serious incidents unfold.

## Sources

- [Cyber Security Breaches Survey 2025, Department for Science, Innovation and Technology](https://www.gov.uk/government/statistics/cyber-security-breaches-survey-2025/cyber-security-breaches-survey-2025)

## How we can help

- [Penetration Testing](https://cybersecuritylondon.com/penetration-testing)
- [Vulnerability Management](https://cybersecuritylondon.com/vulnerability-management)

## Contact

Email enquiry@cybersecuritylondon.com or use the enquiry form at https://cybersecuritylondon.com/#contact. Cyber Security London, The Leather Market, London Bridge, London.