UNiComp.iR

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();
}

private void Form1_Load(object sender, EventArgs e)
{
    MessageBox.Show( Class4.ConvertIP2HostName("127.0.0.1"));
    MessageBox.Show(Class4.HostNameToIP("laptop"));
}