Get all sequences of labels for a domain. This finds "paypal.com" in "paypal.com.hacker.net".

GetDomainSequences

/// <summary>
/// Get all sequences of labels for a domain. 
/// This finds "paypal.com" in "paypal.com.hacker.net".
/// </summary>
/// <remarks>
/// Inner loop is reversed so that returned list looks like a depth first search.
/// This is just a personal preference.
/// </remarks>
/// <param name="domain"></param>
/// <returns></returns>
public static IEnumerable<string> GetDomainSequences(string domain)
{
  var parts = domain.Split('.').ToArray();
  var labels = new List<String>(Enumerable.Range(0, parts.Length).Sum(x => x + 1));
  for (var i = 0; i <= parts.Length; i++)
  {
      for (var j = parts.Length - i; j >= 1; j--)
      {
          labels.Add(String.Join(".", parts.Skip(i).Take(j)));
      }
  }
  return labels;
}