What is the localhost IP address?

What loopback means

Every computer has a virtual network interface called loopback. Anything sent to the loopback address is delivered straight back to the same machine without touching a real network. Developers use it to run and test servers privately: a dev server on localhost is reachable only from that computer.

127.0.0.1, ::1 and localhost

• 127.0.0.1: the usual IPv4 loopback address. Any address from 127.0.0.0 to 127.255.255.255 also loops back.
• ::1: the IPv6 loopback address.
• localhost: a name that resolves to one or both. Some tools try ::1 first, so a server listening only on 127.0.0.1 can appear unreachable when you type localhost. Using 127.0.0.1 directly avoids that.

0.0.0.0 is different

When a server listens on 0.0.0.0, it accepts connections on all network interfaces, not only loopback. That is what you need for access from other devices on your network or from outside a container. As an address to connect to, 0.0.0.0 is not meaningful, and you should use a real IP instead.

# reachable only from this machine
app.listen(3000, "127.0.0.1");
# reachable from other devices and from outside a container
app.listen(3000, "0.0.0.0");

Containers and virtual machines

Inside a Docker container, localhost is the container. If your app in a container calls localhost:5432 expecting the database on your laptop, it finds nothing. With Docker Desktop, the special name host.docker.internal reaches the host. Between containers in Compose, use the service name, such as db:5432.

Testing loopback

To confirm something is listening on loopback, request it directly with curl http://127.0.0.1:3000. If that works and http://localhost:3000 does not, the server listens on IPv4 only and your tool tried IPv6 first. Telling the server to listen on both, or using the numeric address, resolves it.

Security

Binding a development database or admin tool to 127.0.0.1 keeps it off the network. Binding it to 0.0.0.0 on a laptop in a café or a cloud server makes it reachable by others. Choose deliberately.

Production code that calls localhost works only where the service runs on the same machine. In a browser, localhost is the visitor's own device.

What a scan can and cannot tell you

WebAuditOS scans public sites, so it never sees localhost. After deployment, it reports failed requests and console errors, which is where a frontend still calling a loopback address shows up.

Short answers

Is localhost always 127.0.0.1? Usually, and ::1 for IPv6.

Can someone else access my 127.0.0.1? No. It only loops back on your machine.

What is the difference between 127.0.0.1 and 0.0.0.0? 127.0.0.1 is loopback only. Listening on 0.0.0.0 means all interfaces.

Why does localhost work but 127.0.0.1 not, or the reverse? The server listens on only one of IPv4 or IPv6.