What is Nmap and how to check the ports on your own network?

Published on 2026-10-17 by frfrand

Qué hace Nmap, cómo interpretar puertos abiertos, cerrados y filtrados, y cómo realizar una comprobación sencilla en una red autorizada.

You installed an application on your server and opened a port to test it. Two weeks later, you don't even remember which services are still accessible. Nmap is a tool that allows you to explore computers and ports on a network to understand what is responding. It is used in inventories, diagnostics, and security audits.

It is not a hacking button. Nor is it a guarantee that a server is protected.

What is a port and what does Nmap see?

An IP address helps find a computer. A port identifies a possible communication path with a service on that computer.

For example, it is common for a website to receive HTTPS traffic via port 443, while SSH uses port 22. This does not force all servers to use those numbers and does not mean that every service has to be exposed.

Nmap can send network requests and observe the responses. This way, it reports whether certain ports appear open, closed, or filtered.

A simple and authorized example

Suppose you have a lab computer with IP 192.168.1.10. You want to check which common TCP ports are available.

From another computer on that same network, and only if you have authorization to evaluate it, you can run:

nmap 192.168.1.10

The default scan checks a set of common TCP ports. If 22/tcp open ssh appears, it means Nmap observed an open port and associated it with the SSH service. It does not mean it discovered a password or that it is possible to log in.

You can limit the scope to a specific port:

nmap -p 443 192.168.1.10

This is enough to start understanding the difference between an accessible service and one that does not respond as expected.

What do open, closed, and filtered mean?

Open: the scan found indications that a service accepts connections on that port.

Closed: the computer responds, but there does not appear to be a service listening there.

Filtered: there is some obstacle that prevents determining the port status with certainty; it can be a firewall or a network rule.

The result depends on where the test is performed. A service can be open within the private network and inaccessible from the Internet. That is why it makes sense to check from the point of view of the client that actually needs to connect.

Does an open port mean they can hack me?

Not necessarily. A public website needs to receive connections. The problem appears when you expose services that should not be public, use vulnerable software, or configure permissions and authentication insecurely.

An open port is a clue to review, not a sentence that your server is compromised.

Common mistakes when using Nmap

The first is analyzing third-party networks without authorization. That a computer responds over the Internet does not give you permission to audit it.

The second is interpreting a scan as a complete evaluation. Nmap helps you see services, but it does not replace updates, access controls, configuration reviews, and vulnerability analysis.

The third is assuming that a filtered port guarantees security. A system can have other problems that are not detected by looking at ports.

How to apply it in ArduMaker?

If you manage a VPS from ArduMaker, it's worth being clear about which services should receive public connections. For example, publishing HTTPS for an application does not require opening PostgreSQL to the entire Internet. Checking your own ports can complement firewall configuration.

In summary: Nmap helps answer a very useful question: "What services appear accessible from here?". It does not replace a full audit, but it is a good tool to stop guessing.

Source: Nmap official manual on ports.