Getting an error "index was outside the bounds of the array" in C#

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;

using System.Windows.Forms;
using System.Data.SqlClient;

public partial class Login : System.Web.UI.Page
{
    
    SqlConnection con;
    
    protected void Page_Load(object sender, EventArgs e)
    {
        ButtLogon.Focus();
        con = new SqlConnection(ConfigurationManager.ConnectionStrings["sfs"].ConnectionString);
        
    }
    public string GetIpAddress()
    {
        string hostname=System.Net.Dns.GetHostName();
        System.Net.IPAddress[] ips = System.Net.Dns.GetHostAddresses(hostname);
        string Org_IpAddress = ips[1].ToString();

//after executing above statement program jumps up to the catch block n raise d exception

        return Org_IpAddress;
    }

    public bool ValidStudent(string course_string)
    { 
        string ip=GetIpAddress();
        bool b=false;
        SqlCommand cmd = new SqlCommand("select ip_address from student_login where course_string='" + course_string + "'",con);
        con.Open();
        SqlDataReader dr = cmd.ExecuteReader();
        if (dr.Read())
        {
            if (dr[0].ToString() == ip)
            {
                b = true;
            }
        }
        con.Close();
        return b;
    }

    public SqlDataReader FetchDetails_frmString(string s)
    {
        string query = "select * from string_gen where string='" + s.Trim() + "'";
        SqlCommand cmm = new SqlCommand(query, con);
        con.Open();
        SqlDataReader dr = cmm.ExecuteReader();
        return dr;        
    }
    public int  StoreIpAddress(string ip,string course_string)
    {
        int i = 0;
        SqlCommand cmd = new SqlCommand("insert into student_login values('" + course_string + "','" + ip + "')");
        cmd.Connection = con;
        con.Open();
        i = cmd.ExecuteNonQuery();
        con.Close();
        return i;
    }
   string s;
    protected void ButtLogon_Click(object sender, EventArgs e)
    {
        try
        {
            bool check = ValidStudent(TBRNo.Text);
            if (check == false)
            {
                SqlDataReader dr = FetchDetails_frmString(TBRNo.Text);
                DataTable dt = new DataTable();
                
                dt.Load(dr);
                Session["dr"] = dr;
                Session["FetchDetails_frmString"] = dt;
                con.Close();
                if (dt.Rows.Count > 0)
                {
                    string ip = GetIpAddress();
                    int i = StoreIpAddress(ip, TBRNo.Text);
                    if (i > 0)
                    {
                        ClientScript.RegisterStartupScript(GetType(), "Onload", "alert('Login Successful!')", true);
                        Response.Redirect("~/Student/home.aspx");
                    }
                    else
                    { 
                        
                    }
                   
                }
                else
                {
                    ClientScript.RegisterStartupScript(GetType(), "Onload", "alert('Access Denied.....This string does not exist')", true);
                }
            }
           else
            {
                ClientScript.RegisterStartupScript(GetType(), "Onload", "alert('You have Already login this Website')", true);
            }
        }
        catch (Exception o)
        {
            Label8.Text = o.Message;
            ClientScript.RegisterStartupScript(GetType(), "Onload", "alert('Access Denied.....Some Problem occured')", true);
        }
        finally 
        { 
            con.Close();         
        }
                }
    protected void TBRNo_TextChanged(object sender, EventArgs e)
    {

    }
}