0.0.0.0 vs 127.0.0.1

The difference between 0.0.0.0 and 127.0.0.1

0.0.0.0: This is a non-routable meta-address used to designate an invalid, unknown, or non-applicable target. In the context of servers, 0.0.0.0 means "all IPv4 addresses on the local machine." If a server is set to listen on 0.0.0.0, it will be reachable at any IPv4 address that the machine happens to have. This makes it useful for services that should be accessible from any network. For example, if your computer is connected to multiple networks (e.g., Ethernet and Wi-Fi), a service listening on 0.0.0.0 can accept connections from both.
127.0.0.1: This is the loopback address for IPv4. It is used to establish an IP connection to the same machine or host. This address is commonly used for testing network software. When you connect to 127.0.0.1, you are effectively connecting to "yourself." In the context of a server, listening on 127.0.0.1 means the service will only be reachable from the local machine. It is not accessible from other machines on the network.

So typically, will people set up a web server listening on a host rather than 0.0.0.0 or 127.0.0.1?

In practice, the choice of which IP address a web server should listen on—be it 0.0.0.0, 127.0.0.1, or a specific host IP—depends on the desired accessibility and security considerations for the web service. Here's how these options are typically used:

Listening on 0.0.0.0: This is a common setup for servers intended to be accessible from any network interface on the server. By binding to 0.0.0.0, the server listens for incoming connections on all IPv4 addresses that the server possesses. This is particularly useful when the server is part of a larger network or is meant to be accessible from the internet. It's a standard configuration for production web servers.
Listening on 127.0.0.1: Binding to the loopback address (127.0.0.1) is used when the service should only be accessible from the local machine itself. This setup is common for development purposes, where developers run a web server on their machines that they don't want to expose to the external network. It's also used for security reasons when a service should not be accessible from outside the host.
Listening on a Specific Host IP: In some scenarios, a server might be configured to listen on a specific IP address. This is used when you want to restrict the service to a particular network interface. For example, if a server has multiple network interfaces (e.g., one public and one private), you might configure the web server to listen only on the private network for internal access.
Listening on an IPv6 Address: With the growing adoption of IPv6, web servers might also listen on an IPv6 address, either in addition to or instead of IPv4 addresses.

In summary, the choice of listening address depends on the use case:

Use 0.0.0.0 for general accessibility across all network interfaces.
Use 127.0.0.1 for local testing or security reasons.
Use a specific IP address for controlled access on a specific network interface.
Consider IPv6 addresses for modern network compatibility.

For most production environments where services are intended to be widely accessible, listening on 0.0.0.0 (or the appropriate IPv6 equivalent) is typically the preferred configuration. However, always consider security implications and configure firewalls and other protective measures appropriately.