Request.UserHostAddress == “::1” ， 是什么原因？

using System;
using System.Net;

public class IPNetworking
{
  public static string GetIP4Address()
  {
    string IP4Address = String.Empty;

    foreach (IPAddress IPA in Dns.GetHostAddresses(HttpContext.Current.Request.UserHostAddress))
    {
      if (IPA.AddressFamily.ToString() == "InterNetwork")
      {
        IP4Address = IPA.ToString();
        break;
      }
    }

    if (IP4Address != String.Empty)
    {
      return IP4Address;
    }

    foreach (IPAddress IPA in Dns.GetHostAddresses(Dns.GetHostName()))
    {
      if (IPA.AddressFamily.ToString() == "InterNetwork")
      {
        IP4Address = IPA.ToString();
        break;
      }
    }

    return IP4Address;
  }
}


string ClientIP = GetIP4Address();