using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var urls = new string[] {

            "https://a.com?download=1.mp3",     "https://a.com?download=2.mp3", 

            "https://a.com?download=3.mp3",     "https://a.com?download=4.mp3", 

            "https://b.nl?download=alpha.mp3",  "https://b.nl?download=beta.mp3", 

            "https://b.nl?download=gamma.mp3",  "https://b.nl?download=delta.mp3", 

            "https://c.es?download=i.mp3",      "https://c.es?download=ii.mp3", 

            "https://c.es?download=iii.mp3",    "https://c.es?download=iv.mp3" };

        var x = urls

            //convert the urls to uri's

            .Select(u => new Uri(u))

            //create groups of unique hosts

            .GroupBy(u => u.Host)

            //select them back together into a single list

            .SelectMany((group, groupIndex) => group.Select((item, index) => new { Index = index, GroupIndex = groupIndex, Value = item }))

            //order by the group item index

            .OrderBy(u => u.Index)

            //order by the group index

            .ThenBy(u => u.GroupIndex)

            //show position (or one could select u.Value.ToString() for just the URL)

            .Select((u, position) => new { Position = position, Index = u.Index, GroupIndex = u.GroupIndex, Value = u.Value });

        x.ToList().ForEach(u => Console.WriteLine(u.Value));

    }

}