How to determine whether an IP is from the same LAN programatically in .NET C#

Question

I know that if an IP falls outside the Subnet Mask + Local IP rules, it will be only reachable through an Gateway. The problem is that I don't know how to obtain the local IP address, neither the local subnet mask, programatically using .NET. Any of you can help me?

I will use this information to squeeze the maximum performance from my batch SQL insertion queue. If the SQL server falls in the same subnet, then it will use an algorithm optimized for mininum latency, otherwise I´ll use one optimized for high latency.

Answer 1

This will get you the host name and the IP address. I'm assuming you know the IPs in your LAN so you should be able to determine if the IP address is outside the LAN that way:

// Get the host name of local machine.
strHostName = Dns.GetHostName();
Console.WriteLine("Local Machine's Host Name: " + strHostName);

// Using the host name, get the IP address list.
IPHostEntry ipEntry = Dns.GetHostEntry(strHostName);
IPAddress[] addr = ipEntry.AddressList;

for (int i = 0; i < addr.Length; i++)
{
Console.WriteLine("IP Address {0}: {1} ", i, addr[i].ToString());
}

Answer 2

Hi Vernicht,

You can use the classes inside the System.Net.NetworkInformation namespace (introduced in .NET 2.0):

        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
                    );
            }
        }

Answer 3

There is an alternative way using the NetworkInformation class:

public static void ShowNetworkInterfaces()
{
 // IPGlobalProperties computerProperties = IPGlobalProperties.GetIPGlobalProperties();
 NetworkInterface[] nics = NetworkInterface.GetAllNetworkInterfaces();

 if (nics == null || nics.Length < 1)
 {
  Console.WriteLine("  No network interfaces found.");
  return;
 }

 Console.WriteLine("  Number of interfaces .................... : {0}", nics.Length);
 foreach (NetworkInterface adapter in nics)
 {
  IPInterfaceProperties properties = adapter.GetIPProperties();
  Console.WriteLine();
  Console.WriteLine(adapter.Description);
  Console.WriteLine(String.Empty.PadLeft(adapter.Description.Length,'='));
  Console.WriteLine("  Interface type .......................... : {0}", adapter.NetworkInterfaceType);
  Console.WriteLine("  Physical Address ........................ : {0}", adapter.GetPhysicalAddress().ToString());
  string versions ="";

  // Create a display string for the supported IP versions.
  if (adapter.Supports(NetworkInterfaceComponent.IPv4))
  {
   versions = "IPv4";
  }
  if (adapter.Supports(NetworkInterfaceComponent.IPv6))
  {
   if (versions.Length > 0)
   {
    versions += " ";
   }
   versions += "IPv6";
  }
  Console.WriteLine("  IP version .............................. : {0}", versions);
  UnicastIPAddressInformationCollection uniCast = properties.UnicastAddresses;
  if (uniCast != null)
  {
   foreach (UnicastIPAddressInformation uni in uniCast)
   {
    Console.WriteLine("  Unicast Address ......................... : {0}", uni.Address);
    Console.WriteLine("     Subnet Mask  ......................... : {0}", uni.IPv4Mask);
   }
  }
 Console.WriteLine();
 }
}

The code sample is a mashup form the examples provided by Msdn, simplified to only show the information you probably need.

EDIT: Took me too long (too many things at the same time :) ) to make this post, and Mitch beat me to it :)