Network.HavePublicAddress

Description

Check if this machine has a public IP address.

It checks all the network interfaces for IPv4 public addresses and returns true if one address is found.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void OnGUI() {
        if (GUILayout.Button("Start Server")) {
            bool useNat = !Network.HavePublicAddress();
            Network.InitializeServer(32, 25002, useNat);
            MasterServer.RegisterHost("MyUniqueGameType", "JohnDoes game", "l33t game for all");
        }
    }
}