IP Address

You might say

Why can I open the page on my computer but others cannot?

A network-layer address used to locate an interface and route dataIn local development, 127.0.0.1 is the current device's loopback interface; a LAN address is reachable only within its routed network. Sharing localhost with a coworker still points to their own computer. A public IP alone does not mean anyone can connect: routing, ports, service listening, and access rules also matter.

Know first

Port

IP AddressIP

Who can reach each kind of address

localhost:3000

Loopback addressPoints to the device making the request

Dev service on the computerService computer · 192.168.1.20:3000

Computer browser →localhost:3000 → the computer's own loopback interface

Can open: this example service runs on this computer

Phone browser →localhost:3000 → the phone's own loopback interface

Fails: this example phone is not running the dev service

localhost always points to the device making the request. In this example the dev service runs on the computer and listens on its local port; the phone is not running it, so the phone's localhost cannot reach the computer.

The dev service runs on the computer at 192.168.1.20:3000 on the LAN; localhost points to whichever device makes the request.

Address scope decides who can reach it

127.0.0.1: only this machine: localhost and 127.0.0.1 mean this very computer; anyone else using the address reaches their own machine.

LAN address: routed within a network: Private addresses such as 192.168.x.x and 10.x.x.x can be routed within their network; the service must also listen on a reachable interface, and firewall and network rules must allow the connection.

A public address: potentially reachable from the internet: A public address can be routed from the internet, but connections still depend on ports, service listening, firewalls, and other access rules; a domain name resolves to an address.

You can say this to an AI Agent

The local service runs on localhost:3000 and my colleague cannot reach it. Tell me this machine's LAN address and confirm the dev server listens on all interfaces rather than localhost only; if public access is needed, explain where to deploy.

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1Domain→2DNS→3Port→4Web Hosting→

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Your project runs on localhost:3000 and a colleague wants to access your dev server. What address should you give?

A. Just give localhost:3000; the same address should workB. The port 3000 alone is enoughC. localhost points only at their own computer; use a LAN address or deploy publicly