Note: this post is based on experience with Linux.

We’ve all been there - you’re working on a server on your local machine and need to show it to someone else on your local network. You grab your machine’s IP using netstat, ifconfig, or similar and send it over. And… they can’t connect.

If there isn’t an issue with your firewall settings, and you Google the right thing, you may find a StackOverflow post telling you that your server has to bind on 0.0.0.0 rather than 127.0.0.1. But what does this mean and how does it work?

The Loopback Interface

Network Interfaces are objects within the kernel that represent an entry point for the host on a particular network for sending/receiving packets. They can be used to represent physical Network Interface Cards but they can also be virtualised for other purposes.

The purpose of the loopback interface, lo, is to give the host the ability to efficiently send packets to itself. The kernel implements this by returning packets straight back to the interface without going any deeper into the kernel’s networking stack.

When your server process binds to 127.0.0.1 it is bound to the lo Network Interface. This is why you cannot connect to it from your local network - the lo interface can only be reached from your local machine.

0.0.0.0 Special Case

If you can’t connect to the server process from your local network when it is bound to 127.0.0.1, what should you do instead? One option is to bind to your machine’s address on your local network (192.168.1.12 or whatever). However, it can be quite tedious to work out which interface is connected to your local network and the IP that has been assigned to that interface.

Another option is to bind to an address that can be routed to from all Network Interfaces. This is what happens when you bind to 0.0.0.0.

0.0.0.0 is a non-routable address that, by convention, will receive packets from every Network Interface on the host.

So, the next time you want to host something on your local network, remember to bind to 0.0.0.0.