nixos-config/hosts/falkenstein-1/modules/networks/default.nix

{ ... }:
{
  networking = {
    hostName = "falkenstein-1";
    useNetworkd = true;
    enableIPv6 = true;
  };
  systemd.network = {
    enable = true;
    networks."10-loopback" = {
      matchConfig.Name = "lo";
      linkConfig.RequiredForOnline = false;
    };
    networks."10-wired" = {
      matchConfig.Name = "ens3";
      networkConfig = {
        DHCP = "ipv4";
        IPv6AcceptRA = "yes";
        Address = "2a01:4f8:c012:49de::1/64";
        Gateway = "fe80::1";
      };
    };
  };
}