# Port

URL: https://softwaredictionary.org/terms/network-port
Category: Networking
Last updated: 2026-09-30

In short: A port is a number from 0 to 65535 that identifies a specific program or service on a device, so traffic reaching an IP address gets to the right application.

## What is a port in networking?

In networking, a port is a number that identifies a specific program or service running on a computer. An IP address gets data to the right machine, and the port number gets it to the right application on that machine. Port numbers range from 0 to 65535 and are used by both TCP and UDP.

When a server program starts, it listens on a port, such as a web server waiting on port `443`. A client connecting to it uses a temporary port of its own, called an ephemeral port, so each connection is identified by the combination of both IP addresses and both ports. Ports 0 to 1023 are well-known ports reserved for common services, such as `22` for SSH, `53` for DNS, `80` for HTTP, and `443` for HTTPS.

Think of an IP address as the street address of an apartment building and the port as the apartment number. The mail carrier gets the letter to the building, and the apartment number decides who receives it. Developers meet ports constantly, for example when a local development server runs at `localhost:3000` or when a firewall rule allows only port `443`.

A network port is not the same as a physical port, such as a USB or Ethernet socket on a device; a network port is purely a number in software. The same word is also used for adapting software to run on another platform, which is unrelated. Only one program can listen on a given port and address at a time, which is why starting a second one fails with errors like `EADDRINUSE` (address already in use).

## Key takeaways

- A port number identifies a specific program or service on a device.
- Port numbers range from 0 to 65535, and TCP and UDP each have their own set.
- Well-known ports include `22` (SSH), `53` (DNS), `80` (HTTP), and `443` (HTTPS).
- A connection is identified by source IP, source port, destination IP, and destination port.
- Only one program can listen on the same port and address at a time.

## Example: Listening on a port and connecting to it

```bash
# Start a simple web server listening on port 8000
python3 -m http.server 8000

# In another terminal, send a request to that port
curl http://localhost:8000/

# List programs listening on TCP ports (Linux)
ss -tlnp
```

## Frequently asked questions

**What is the difference between an IP address and a port?**

An IP address identifies a device on a network, while a port identifies a specific program or service on that device. Together, as in `192.168.1.10:443`, they point to one exact destination.

**What does localhost:3000 mean?**

`localhost` refers to your own computer, and `3000` is the port where a program, often a development server, is listening. Opening that address in a browser connects to that program on your machine.

**What does it mean when a port is open?**

An open port has a program listening on it, and the firewall allows traffic to reach it. Closing unused ports reduces the number of ways an attacker can reach a machine.

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Software Dictionary: https://softwaredictionary.org/ · https://softwaredictionary.org/llms.txt