Added new simple-network.md page.

doc/src/simple-network.md

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+# A simple network setup
+
+While networking infrastructure is out of scope for the **microvm.nix**
+flake, here is some guidance for providing the MicroVMs on your NixOS
+machine with internet access.
+
+Because we already use systemd for MicroVM startup, let's pick
+`systemd-networkd`:
+```nix
+networking.useNetworkd = true;
+```
+
+## A bridge to link TAP interfaces
+
+To make your MicroVM reachable, the host will place its Ethernet port (`eno1`)
+on a bridge (`br0`). This bridge will have the microVM's TAP interface attached
+to it - directly placing the microVM on your local network.
+
+Note that the addresses provided below are examples and you must adjust these
+for your network settings. Also note that the `eno1` must be paired on the
+bridge with the `vm-*` TAP interfaces that you will specify in the microVM
+definition.
+
+```nix
+systemd.network.enable = true;
+
+systemd.network.networks."10-lan" = {
+  matchConfig.Name = ["eno1" "vm-*"];
+  networkConfig = {
+    Bridge = "br0";
+  };
+};
+
+systemd.network.netdevs."br0" = {
+  netdevConfig = {
+    Name = "br0";
+    Kind = "bridge";
+  };
+};
+
+systemd.network.networks."10-lan-bridge" = {
+  matchConfig.Name = "br0";
+  networkConfig = {
+    Address = ["192.168.1.2/24" "2001:db8::a/64"];
+    Gateway = "192.168.1.1";
+    DNS = ["192.168.1.1"];
+    IPv6AcceptRA = true;
+  };
+  linkConfig.RequiredForOnline = "routable";
+};
+```
+
+Now that the host is configured, you can define a microVM to have a static IP
+address with:
+
+```nix
+microvm = {
+  #...add additional microVM configuration here
+  interfaces = [
+    {
+      type = "tap";
+      id = "vm-test1";
+      mac = "02:00:00:00:00:01";
+    }
+  ];
+};
+
+systemd.network.enable = true;
+
+systemd.network.networks."20-lan" = {
+  matchConfig.Type = "ether";
+  networkConfig = {
+    Address = ["192.168.1.3/24" "2001:db8::b/64"];
+    Gateway = "192.168.1.1";
+    DNS = ["192.168.1.1"];
+    IPv6AcceptRA = true;
+    DHCP = "no";
+  };
+};
+```
+
+For more networking options - such as port forwards for a single IP address,
+see the [advanced networking](./advanced-network.md) page.