# IP Addresses and Ports

An IP address finds the computer, and a port finds the program on it. Meet localhost, the ports the web uses, and the ports your own apps listen on.

Novice · Web Development · Updated November 3, 2026

## After this lesson you can

- Explain what an IP address, localhost and a port are
- Find the port a program listens on, such as a web app's port in launchSettings.json

## Key takeaways

- An IP address finds the computer, and a port finds the program on it, like a phone number and an extension.
- A domain name such as `google.com` is a nickname for an IP address.
- `localhost`, or `127.0.0.1`, is your own computer's address for itself.
- Websites listen on port 80 for `http` and 443 for `https`, and browsers hide those default ports in the address.
- Your web app's port, such as 5204, is in `Properties/launchSettings.json`, and each new project gets its own.

To send a message to another program, you need to know two things: which **computer** it's on, and which **program** on that computer.

## IP addresses find the computer

Every computer on a network has an **IP address** (Internet Protocol address), unique on its network. It's four numbers from 0 to 255, separated by dots:

```text
142.250.191.36
```

People don't remember numbers well, so we use **domain names** instead, such as `google.com`. A domain name is a nickname for an IP address: your computer looks up the number behind the name, and sends the message to the number.

![A browser reaching a server across the internet by its IP address](https://fsdtrainingmedia.blob.core.windows.net/public/images/2026/09/8cee2568223e4eb3ac8cbaf73e5e6921.png "A browser reaching a server across the internet by its IP address")

## localhost is your own computer

When both programs are on the same computer, the address is always `127.0.0.1`, and its nickname is **localhost**. You've already used it: your connection string says `Server=localhost`.

![localhost or 127.0.0.1: the computer's address for itself](https://fsdtrainingmedia.blob.core.windows.net/public/images/2026/09/67f1235812e043f6881995fc7869d88c.png "localhost or 127.0.0.1: the computer's address for itself")

## Ports find the program

One computer runs many programs that listen for messages, so the address alone isn't enough. Think of an office with one phone number and many extensions: the IP address is the phone number, and the **port** is the extension. Each listening program has its own port number.

| Program | Port it listens on |
|---|---|
| A website, over `http` | 80 |
| A website, over `https` | 443 |
| SQL Server, on a network | 1433 |
| The web app you'll make next, while you build it | a number such as 5204, chosen when you create it |

On your own computer, SqlClient can take a shortcut to SQL Server that skips the port, which is why your connection string doesn't name one. A database in the cloud is always reached through its port.

## Ports in web addresses

A port goes after the address, following a colon. Browsers hide the default ports, so these pairs are the same address:

```text
http://www.example.com        is   http://www.example.com:80
https://www.example.com       is   https://www.example.com:443
```

While you build a web app, it runs on your own computer, on a port of its own, so that several apps can run at once without getting in each other's way:

```text
http://localhost:5204
```

ASP.NET Core writes the port in the project's `Properties/launchSettings.json` file when you create it:

```json
"http": {
  "commandName": "Project",
  "launchBrowser": true,
  "applicationUrl": "http://localhost:5204"
}
```

Your number will be different: each new project gets its own.

## Check yourself

2 questions to try on the lesson page.

## So what

An address and a port are all it takes to reach any program, on your computer or across the world. When a browser can't reach your app, or your app can't reach its database, those are the first two things to check.

## Key terms

- [IP Address](https://www.ask-codey.com/glossary/ip-address)
- [Domain Name](https://www.ask-codey.com/glossary/domain-name)
- Localhost and Ports
- launchSettings.json

## Cheat sheets

- [Network Communication, IP Addresses and Ports](https://www.ask-codey.com/cheat-sheets/network-communication-in-csharp-an-overview-cheat-sheet)

Source: https://www.ask-codey.com/lessons/network-communication-in-csharp-ip-addresses-and-ports