using System.Net;
public string ConvertIP2HostName(string ip)

{

    IPHostEntry iphost = Dns.GetHostEntry(ip);

    return iphost.HostName;

}
public string HostNameToIP(string hostName)

{

    IPHostEntry iphost = System.Net.Dns.GetHostEntry(hostName);

    IPAddress[] addresses = iphost.AddressList;

    StringBuilder addressList = new StringBuilder();

    foreach (IPAddress address in addresses)

    {

        addressList.AppendFormat("IP Address: {0};", address.ToString());

    }

    return addressList.ToString();

}
نحوه ی استفاده :
private void Form1_Load(object sender, EventArgs e)

{

    MessageBox.Show( Class4.ConvertIP2HostName("127.0.0.1"));

    MessageBox.Show(Class4.HostNameToIP("laptop"));

}


منبع : "C#‎ 3.0 Cookbook™, Third Edition"