Port (network port)
A port is a numbered doorway on a device that a particular kind of network traffic uses, for example secure web pages usually arrive on port 443. Letting internet traffic reach an inside system is called opening a port. Cyber Essentials requires a documented, approved business case for each one, asked at A4.5 and A4.8.
- Term
- Port
- Also known as
- Network port, TCP port, UDP port, Open port
- Category
- Networks and firewalls
- Requirement area
- 1. Firewalls
- Reviewed against
- Requirements v3.3
What does the term port mean in Cyber Essentials?
A port is a numbered doorway on a device that a particular kind of network traffic uses, for example secure web pages usually arrive on port 443.
How does port apply to Cyber Essentials?
In plain terms
Picture an office building with one street address and hundreds of numbered doors. The address gets visitors to the building. The door number decides which department they reach. On a network, the IP address is the street address and the port is the door number. Email, web pages and remote desktop each tend to use their own door.
An open port is a door that answers when someone knocks from the internet.
What Cyber Essentials asks
The question set uses the phrase directly. The guidance to A4.5 and A4.8 says your firewall may be set up "to allow a system on the inside to become accessible from the internet (for example: a VPN server, a mail server, an FTP server, or a service that is accessed by your customers). This is sometimes referred to as 'opening a port'. You need to show a business case for doing this because it can present security risks."
So every open port needs a reason, an approver and a record (A4.8), and should be removed when it is no longer needed (A4.6).
For Cyber Essentials Plus, test case 1 scans all your internet addresses "on the recommended set of TCP and UDP ports" and judges each service it finds.
Common mistakes
- Ports opened by a supplier for remote support and never closed.
- Opening a port for remote desktop straight to the internet with a password only.
- No record of why a port is open, which leaves A4.8 unanswerable.
See the firewall requirements and Test 1: the external vulnerability scan. Related: port forwarding, firewall rule.
Related terms
- IP addressAn IP address is the number that identifies a device or internet connection on a network, so that traffic can be delivered to the right place.
- Remote Desktop Protocol (RDP)Remote Desktop Protocol (RDP) is Microsoft's way of letting someone see and control a Windows computer or server from another device over a network.
- Port forwardingPort forwarding is a router setting that sends a particular kind of traffic arriving from the internet on to one chosen device inside your network, such as a CCTV recorder.
- Firewall ruleA firewall rule is a single instruction on a firewall that says which traffic to allow or block, based on where it comes from, where it is going and what type it is.
- RouterA router is the box that connects your office or home network to the internet and passes traffic between the two.
- Software firewallA software firewall is a firewall configured on a device that works like a boundary firewall but only protects the single device on which it is configured.
Frequently asked questions
A port is a numbered doorway on a device that a particular kind of network traffic uses, for example secure web pages usually arrive on port 443.
In the 2026 Danzell question set, port comes up at A4.5, A4.8. Read those questions against the definition above so your answers use the term the way the assessor will.
Port relates to 1. Firewalls in the NCSC's Cyber Essentials: Requirements for IT Infrastructure v3.3. Read that section for the control your answers must show.
Would you pass the 2026 question set?
Answer 24 yes or no questions, built by the assessors who mark Cyber Essentials, and get a readiness score in about five minutes, with any auto-fail questions you miss flagged first in your results.