What Is Localhost? (And Why Your Friend Can't Open Your Link)

You've got your app running. The terminal says:

Local: http://localhost:3000

You open it — it works. You send the link to a friend — "This site can't be reached." What happened?

Localhost means "this computer"

localhost is a special name that always means the computer you're using right now. When you open localhost:3000, your browser talks to your own machine. When your friend opens localhost:3000, their browser talks to their machine — where your app isn't running.

So a localhost link can never be shared. It's like telling someone "come to my house" by writing "here" on the envelope.

You'll also see 127.0.0.1 — that's the numeric address for the same thing.

What's the :3000? Ports

One computer can run many programs that talk over the network. Ports keep them apart, like apartment numbers in a building. Your app listens on one port — 3000, 5173, 8000, and 8080 are common for development — and the browser needs to know which one to knock on.

A few useful facts:

• Ports 80 and 443 are the defaults for websites (HTTP and HTTPS). That's why real sites don't show a port: https://example.com means port 443.
• "Port already in use" errors mean another program — often an older copy of your app you forgot to stop — is already on that port. Stop it (Ctrl+C in its terminal), or run your app on a different port.

Why your app runs on localhost at all

Running on localhost while you build is a good thing: it's fast, private, and nobody else can see your half-finished work. Development tools deliberately start on localhost for that reason.

To let other people use your app, it has to run on a computer that's reachable from the internet — which is what hosting is. (What Is Web Hosting?)

How to share an app you're working on

Option 1: deploy it

The proper way. Put it on a host and share the real URL. Most AI builders have a Publish button, and many hosts give each version a preview URL you can send to people. (Preview Environments for Every Branch.)

Option 2: same Wi-Fi

If your friend is on the same network, they can sometimes reach your computer by its local network address (something like http://192.168.1.23:3000). Many dev servers print this as a "Network" address. You may need to start the app with a setting that allows it (for Vite, --host). Only works on the same network, and firewalls often block it.

Option 3: a tunnel

A tunnel service gives your localhost a temporary public URL. Tools like Cloudflare Tunnel, ngrok, and others do this. Handy for a quick demo or for testing a webhook from an outside service. Remember: while the tunnel is open, anyone with the link can reach your app — and whatever it can reach on your computer. Close it when you're done.

When localhost sneaks into a live app

Here's the problem that catches many beginners. The app is deployed and live, but something doesn't work — and the cause is a leftover localhost somewhere:

• In the code: fetch('http://localhost:3000/api/orders'). On your machine, that reaches your backend. For a visitor, it tells their browser to connect to their own computer. It fails.
• In "Sign in with Google" settings: the allowed redirect URL is set to localhost, so login breaks on the real site.
• In a payment provider's webhook settings: Stripe is sending "payment succeeded" messages to localhost, which it can't reach.
• In emails: password reset links that point to localhost:3000/reset.

The fix is to search your code and your third-party dashboards for localhost and replace it with your real domain — ideally using an environment variable like APP_URL, so it's right automatically in each place. (What Is an Environment Variable?.)

This is one of the most common reasons apps work on your machine and not online. Why Does My App Work Locally but Not in Production? covers the rest.

The summary

• localhost = the computer you're on. It can't be shared.
• The number after the colon is the port — which program to talk to.
• To share, deploy (best), use the same network, or open a temporary tunnel.
• Before launch, hunt down every localhost in your code and your service settings.

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EasySpawn gives your app a real, shareable URL — with HTTPS and your own domain — and a live URL per branch, so you never have to send anyone a localhost link. See how it works or join the waitlist.

Related: What Is Web Hosting? · Dev, Staging, and Production Explained · What Is an IP Address and a Port?