---
title: What is a Network Port? (and how to monitor one)
description: An IP address gets you to the right machine; a port gets you to the right service on it. Here's what ports are, the common ones, and why monitoring them catches outages a website check misses.
canonical: https://watchfor.io/blog/what-is-a-network-port
---

# What is a Network Port? (and how to monitor one)

2026-02-14 · WatchFor Team · Networking · networking, basics, monitoring

An IP address gets you to the right machine; a port gets you to the right service on it. Here's what ports are, the common ones, and why monitoring them catches outages a website check misses.

You know that an [IP address](/blog/what-is-an-ip-address) points to a machine on the network. But a single machine runs many services at once — a web server, a database, SSH, mail. How does traffic reach the *right* one? **Ports.**

If the IP address is a building's street address, the **port is the room number.** Let's make that concrete.

## What a port is

A **network port** is a number (0–65535) that identifies a specific service on a machine. When you connect to `example.com:443`, you're reaching the host at that IP, on **port 443** — where its web server is listening for HTTPS.

Every networked service **listens** on a port. The combination of IP + port is what actually pinpoints a service.

## The ports you'll meet

A handful of "well-known" ports come up constantly:

| Port | Service | What it's for |
| --- | --- | --- |
| **80** | HTTP | Unencrypted web traffic |
| **443** | HTTPS | Encrypted web traffic |
| **22** | SSH | Secure remote login |
| **25 / 587** | SMTP | Sending email |
| **53** | DNS | Name resolution |
| **3306 / 5432** | MySQL / PostgreSQL | Databases |
| **6379** | Redis | Cache / key-value store |

When someone says "the database is on the default port," they mean one of these.

## Open, closed, or filtered

A port is in one of three states, and the difference matters when debugging:

| State | Meaning |
| --- | --- |
| **Open** | A service is listening and accepting connections |
| **Closed** | Nothing is listening — connection refused (instantly) |
| **Filtered** | A firewall is silently dropping traffic — connection hangs |

That distinction explains a lot: a [connection refused](/blog/connection-refused) is "closed" (instant no), while a hanging timeout is usually "filtered" (a firewall dropping packets).

## Why monitor a port directly?

A website check tells you HTTP is working. But plenty of critical things aren't websites — a database, a mail server, a custom TCP service, a game server. **Port monitoring** opens a real connection to a host and port to confirm the service is actually accepting connections.

This catches a class of problem a page check misses entirely:

- A **database** that stopped accepting connections (even though the website is up).
- A **mail server** whose SMTP port closed.
- A **service that crashed** but the web tier in front of it is fine.

A [TCP check](/blog/tcp-vs-udp) can even time the handshake, so you also catch a port that's *open but slow*.

## A quick how-to

To monitor a port, you point a check at a **host and port** and confirm the connection succeeds:

1. Pick the host and port (e.g. your DB host on `5432`).
2. Have the monitor open a TCP connection on a schedule.
3. Alert if it's refused, filtered (times out), or slow to connect.
4. Check from [multiple locations](/blog/global-monitoring-locations) so a single network hiccup doesn't page you.

## The bottom line

| | In one line |
| --- | --- |
| **What** | A number identifying a specific service on a machine (IP = building, port = room). |
| **Common** | 80/443 web, 22 SSH, 25 mail, 53 DNS, DB ports. |
| **States** | Open (listening), closed (refused), filtered (dropped). |
| **Monitor it** | Confirm the service accepts connections — catches non-web outages. |

Ports are how one machine offers many services without them colliding. And because not everything important is a website, monitoring the right ports catches outages — a dead database, a closed mail port — that a homepage check would never see.

Test a port instantly with the free [port checker](/port-checker), or set up continuous checks with [network monitoring](/docs/monitors/network).

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Canonical page: https://watchfor.io/blog/what-is-a-network-port · All articles: https://watchfor.io/blog/all.md · Site guide: https://watchfor.io/llms.txt