Configuring a static IP address is a fundamental task for any server administrator. A predictable IP address is essential for services, network shares, and remote access. While Debian's installer typically defaults to DHCP, setting a static IP is straightforward.

This guide covers two primary methods: the modern systemd-networkd approach (default in many minimal installs and other distributions) and the traditional "legacy" method using the /etc/network/interfaces file.

Before you begin: Always back up

Before editing any system configuration, create a backup. A simple typo can lock you out of your server, especially if you're working remotely.

Identify which files you're about to change. If you plan to use the legacy method, back up the interfaces file:

If you're using or migrating to systemd-networkd, back up its configuration directory (even if it's empty):

The modern method: systemd-networkd

systemd-networkd is a powerful and flexible networking daemon that is part of the systemd init system. It's the recommended approach for modern Debian systems.

Step 1: Identify your network interface

First, find the name of the network interface you want to configure.

You'll see an output like this. Look for your main ethernet interface (e.g., ens18, eth0, enp0s3).

In this example, our interface is ens18.

Step 2: Create a .network configuration file

systemd-networkd configurations live in /etc/systemd/network/. We'll create a simple file to define our static IP. You can name it anything ending in .network, but a numbered prefix helps control the order of application.

Create the file:

Add the following content, replacing the interface name (ens18) and IP details with your own:

• Address: The IP address followed by the subnet mask in CIDR notation (/16 is equivalent to 255.255.0.0).
• Gateway: Your network's default gateway.
• DNS: One or more DNS servers.

Step 3: Enable and apply settings

First, ensure the legacy networking service is disabled and systemd-networkd is enabled to start on boot:

Now, restart the systemd-networkd daemon to apply your new configuration:

You can verify the new IP address using ip a.

The "legacy" method: /etc/network/interfaces

This is the traditional method used by Debian for many years, relying on the ifupdown package. It's still perfectly functional, especially if you have an older or existing setup.

Step 1: Identify your interface

Just as in the modern method, use ip a to find your interface name (e.g., ens18).

Step 2: Edit /etc/network/interfaces

Open the configuration file with a text editor:

By default, it's likely configured for DHCP:

You need to change this. Comment out or delete the DHCP lines for your interface and add the static configuration:

• auto ens18: Brings the interface up automatically at boot.
• iface ens18 inet static: Defines the interface as statically configured.
• address, netmask, gateway: Your core network details.
• dns-nameservers: This line is crucial. If you don't set DNS here, your server won't be able to resolve domain names. This requires the resolvconf package to be installed (sudo apt install resolvconf).

Step 3: Apply the configuration

To apply the changes, you can restart the networking service:

Alternatively, you can try to bring the interface down and up again (this can be risky over SSH):

Conclusion

While both methods achieve the same goal, systemd-networkd is the recommended path for new Debian installations due to its flexibility and integration with the systemd ecosystem. The "legacy" /etc/network/interfaces method remains a simple and reliable option for existing systems or if you prefer its straightforward syntax.