IP Address

You might say

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

The numeric address of each device on the internet, used to route data to its destinationLocal development addresses like 127.0.0.1 or localhost point exclusively to your current machine, paired with a port for each service. When a site works locally but fails on a coworker's device, you have likely shared a loopback address instead of a LAN or public IP; public deployment is required for open internet access.

Know first

Port

IP AddressIP

Who can reach each kind of address

localhost:3000

127.0.0.1 · Only this machine can reach it

localhost means this very computer; a colleague using this address reaches their own machine.

Listening scope and address type are separate: if the service listens on localhost only, the LAN address fails too.

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: reachable inside one network: Addresses like 192.168.x.x and 10.x.x.x work within the same LAN; devices outside cannot reach them.

Public address: anyone can reach it: Deployed services use public addresses; a domain is just a name resolved to an IP before access.

Quick checkChoose the best answer

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

Just give localhost:3000; the same address should workThe port 3000 alone is enoughlocalhost points only at their own computer; use a LAN address or deploy publicly

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.

Learn next

1Domain→2DNS→3Port→4Web Hosting→

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