using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string ipString = "192.168.0.33";

        // index of periods

        string classCNetworkId = ipString.Substring(0, ipString.LastIndexOf('.'));

        Console.WriteLine($"Class C Network ID: {classCNetworkId}");

        // using string split, then re-join

        classCNetworkId = String.Join('.', ipString.Split('.').Take(3));

        Console.WriteLine($"Class C Network ID: {classCNetworkId}");

        // using RegEx

        classCNetworkId = System.Text.RegularExpressions.Regex.Match(ipString, "^\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}").Value;

        Console.WriteLine($"Class C Network ID: {classCNetworkId}");

    }

}