title: Check if IP is in LAN (behind firewalls and routers)
body: <p>I've been crawling in the web for about 5 hours now and couldn't find a solution for my problem:</p>
  
  <p>My company is developing an educational game and I'm writing an autoupdater for it using Monotorrent. The game will be used in schools, but because most schools only have very weak internet connections there should only be one computer in the network that downloads from a httpseeder, and the others should leech from the one computer that is downloading from the httpseed.</p>
  
  <p>So I get loads of IP-addresses from the tracker and need to filter out only the ones that are in the LAN.</p>
  
  <p>Of course schools are sometimes quite strict with firewalls and there will be loads of routers and switches between some computers in a school.</p>
  
  <p>I've already tried most solutions, things like</p>
  
  <pre><code> NetworkInterface[] interfaces = NetworkInterface.GetAllNetworkInterfaces();
  
      foreach (NetworkInterface iface in interfaces)
      {
          IPInterfaceProperties properties = iface.GetIPProperties();
  
          foreach (UnicastIPAddressInformation address in properties.UnicastAddresses)
          {
              Console.WriteLine(
                  "{0} (Mask: {1})",
                  address.Address,
                  address.IPv4Mask
                  );
          }
      }
  </code></pre>
  
  <p>Or similar techniques only deliver the information of the router/switch/whatever.</p>
  
  <p>So in a nutshell, what I want to do is check if a given IP is accessible via LAN.</p>
  
  <p>I'd really appreciate any help because this feature is the last one remaining :)</p>