Check out example codes for "Get the Default gateway address c#". It will help you in understanding the concepts better.

Code Example 1

[DllImport("iphlpapi.dll", CharSet = CharSet.Auto)]
private static extern int GetBestInterface(UInt32 destAddr, out UInt32 bestIfIndex);

public static IPAddress GetGatewayForDestination(IPAddress destinationAddress)
{
    UInt32 destaddr = BitConverter.ToUInt32(destinationAddress.GetAddressBytes(), 0);

    uint interfaceIndex;
    int result = GetBestInterface(destaddr, out interfaceIndex);
    if (result != 0)
        throw new Win32Exception(result);

    foreach (var ni in NetworkInterface.GetAllNetworkInterfaces())
    {
        var niprops = ni.GetIPProperties();
        if (niprops == null)
            continue;

        var gateway = niprops.GatewayAddresses?.FirstOrDefault()?.Address;
        if (gateway == null)
            continue;

        if (ni.Supports(NetworkInterfaceComponent.IPv4))
        {
            var v4props = niprops.GetIPv4Properties();
            if (v4props == null)
                continue;

            if (v4props.Index == interfaceIndex)
                return gateway;
        }

        if (ni.Supports(NetworkInterfaceComponent.IPv6))
        {
            var v6props = niprops.GetIPv6Properties();
            if (v6props == null)
                continue;

            if (v6props.Index == interfaceIndex)
                return gateway;
        }
    }

    return null;
}

Code Example 2

public static IPAddress GetDefaultGateway()
{
    return NetworkInterface
        .GetAllNetworkInterfaces()
        .Where(n => n.OperationalStatus == OperationalStatus.Up)
        .Where(n => n.NetworkInterfaceType != NetworkInterfaceType.Loopback)
        .SelectMany(n => n.GetIPProperties()?.GatewayAddresses)
        .Select(g => g?.Address)
        .Where(a => a != null)
         // .Where(a => a.AddressFamily == AddressFamily.InterNetwork)
         // .Where(a => Array.FindIndex(a.GetAddressBytes(), b => b != 0) >= 0)
        .FirstOrDefault();
}

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