Why the returns IP address is different when "-2" is added behind address length?

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Few days ago , i posted this question,Instead of get the host IP address in IPv4 format, i got IPv6 format[^]
and i got the answer from other website..

When i use

C#

try
    {
        //iphostname = Dns.GetHostName();  // Resolving Host name
        IPHostEntry ipentry = Dns.GetHostEntry(hostLabel.Text);
        IPAddress[] addr = ipentry.AddressList;// Resolving IP Addresses
        for (int i = 0; i < addr.Length; i++)
        {
            try
            {
                ipLabel.Text=  Convert.ToString(addr[i]) + "\r\n";
            }
            catch
            {
                ipLabel.Text += "IP Address            | " + "\r\n";
            }
        }
    }
    catch
    {
                  | " + "\r\n";
    }
}

i got the IP address in IPv6 format,
but when i use the following code,

<pre lang="xml">
string strHostName = "";
strHostName = System.Net.Dns.GetHostName();
IPHostEntry ipEntry = System.Net.Dns.GetHostEntry(strHostName);
IPAddress[] addr = ipEntry.AddressList;
ipLabel.Text = addr[addr.Length - 2].ToString();</pre>

i got the IP address in IPv4 format, and i noticed that addr.length-2
changed the outcome, but i still dont understand why "-2" change the outcome, does anyone know the reason/magic behind this?

Posted 15-Aug-12 8:52am

asdf9009

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[no name] 15-Aug-12 14:57pm

What magic? What is magical about taking the addr array and getting the element that is the # elements - 2 out of it?

asdf9009 15-Aug-12 14:59pm

to be precise, i dont understand how it works...why IPv4 is shows when "addr.Length -2" and it doesn't show when code in for (int i = 0; i < addr.Length; i++)
{
try
{
ipLabel.Text= Convert.ToString(addr[i]) + "\r\n";
}
catch
{
ipLabel.Text += "IP Address | " + "\r\n";
}
}

[no name] 15-Aug-12 15:06pm

Not knowing what it is that you are seeing and not knowing what "doesn't show" means, from your code I would guess that, ipLabel.Text= Convert.ToString(addr[i]) + "\r\n"; would be your problem. This would only show 1 element out of the array and most likely followed by 2 boxes.

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Philip Stuyck · Answer 1 · 2012-08-15T09:23:00

What you don't seem to understand is that addresslist is in fact a list.
The return value contains multiple values where some addresses are ipv4 and some are ipv6. If you are relying on the fact that the one of length-2 is ipv4 then you are doing poor style of programming.
What you should do instead is iterate through all addresses and then inspect the addressfamily.
http://msdn.microsoft.com/en-us/library/system.net.ipaddress_members(v=vs.80).aspx[^]
als see
http://msdn.microsoft.com/en-us/library/system.net.ipaddress.addressfamily(v=vs.80).aspx[^]
In your code you are iterating through all addresses and the last will be shown on the label. Since you want only ipv4 addresses the first will do so something like this is better

C#

for(int i=0; i< hostInfo.AddressList.Length; i++)
{
            if (addr[i].AddressFamily.ToString() ==  ProtocolFamily.InterNetworkV6.ToString()){ 
              try
              {
                ipLabel.Text=  Convert.ToString(addr[i]) + "\r\n";
              }
              catch
              {
                ipLabel.Text += "IP Address            | " + "\r\n";
              }
              break;
            }
        }