If you write the code, it says that the entry point does not contain the main method.

Severity level code description project file line suppression state
Error The CS5001 program does not contain the appropriate static 'Main' method at the entry point. Project 26 C:\Users\aozaki hinagi\Documents\Visual Studio 2017\Projects\Project 26\Project 26\CSC1 Active

using System;
using System.Collections.General;
using System.Linq;
using System.Text;
using System.Net;

namespace Chap5
{
    class DnsGetHostAddresses
    {
        public static void Main (string[]args)
        {
            // Get Hostname
            string hostName=Dns.GetHostName();

            // Obtain IP Address List
            IPAddress [ ] addresses = Dns.GetHostAddresses (hostName);
            foreach (IPAddress address in addresses)
            {
                Console.WriteLine("Address notation:" + address.ToString());

                // Output the byte array of IP addresses across
                Console.WriteLine(
                    "Byte column: {0}\n", string.Join(", ", address.GetAddressBytes()));
            }

            Console.ReadKey();
        }
    }
}