Added the rest of the configuration. Still some things to add.

modules/networking/default.nix

@@ -0,0 +1,8 @@
+{ ... }:
+
+{
+  imports = [
+    ./firewall.nix
+    ./networking.nix
+  ];
+}

modules/networking/firewall.nix

@@ -0,0 +1,11 @@
+{ config, ... }:
+
+{
+  networking.nftables.enable = true;
+  networking.firewall = {
+    enable = true;
+    allowPing = true;
+    allowedTCPPorts = [ 53 80 443 ];
+    allowedUDPPorts = [ 53 443 ];
+  };
+}

modules/networking/networking.nix

@@ -0,0 +1,52 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.numbus.services.networking;
+in
+
+{
+  options.numbus.services.networking = {
+    ipAddress = mkOption {
+      description = "The IP address that this server will use";
+      type = types.str;
+      example = "192.168.1.100";
+    };
+    interface = mkOption {
+      description = "The interface that this server will use to connect to the network";
+      type = types.str;
+      example = "enp1s0";
+    };
+    routerIpAddress = mkOption {
+      description = "The IP address of the router of your network";
+      type = types.str;
+      example = "192.168.1.1";
+    };
+    dnsServers = mkOption {
+      description = "The list of DNS servers that this server will use";
+      type = types.listOf types.str;
+      default = [ "${cfg.ipAddress}" "9.9.9.9" ];
+      example = [ "${cfg.ipAddress}" "9.9.9.9" ];
+    };
+  };
+
+  networking.hostName = "numbus-server";
+  networking.networkmanager.enable = false;
+
+  # Allow rootless containers to bind to port 53 and up
+  boot.kernel.sysctl."net.ipv4.ip_unprivileged_port_start" = 53;
+
+  networking.bridges.br0.interfaces = [ "${cfg.interface}" ];
+  networking.interfaces."${cfg.interface}".useDHCP = false;
+  networking.interfaces.br0.useDHCP = false;
+  networking.nameservers = ${cfg.dnsServers};
+  networking.interfaces.br0.ipv4.addresses = [{
+    address = "${cfg.ipAddress}";
+    prefixLength = 24;
+  }];
+  networking.defaultGateway = {
+    address = "${cfg.routerIpAddress}";
+    interface = "br0";
+  };
+}