1. Arch installation medium uses systemd-networkd
  2. Edit the config file: vim /etc/systemd/network/20-ethernet.network
  3. Edit the [Network] portion in the file, write Address in CIDR notation.
  4. Remove or comment out DHCP related lines.
  5. Run systemctl restart systemd-networkd.

Example:

[Network]
Address=143.239.81.132/24
Gateway=143.239.81.254
DNS=143.239.1.2

If the .network config file does not exist (for e.g. outside of the installation medium), create it at /etc/systemd/network/20-ethernet.network

Minimally, the whole config file should look something like this:

[Match]
Name=e*

[Network]
Address=143.239.81.132/24
Gateway=143.239.81.254
DNS=143.239.1.2

e* ensure it matches with any ethernet port names, e.g. enp1s0