Related data from one dropdownlist to another in VB.NET

Generally while doing registration in a website. When we choose any city dropdown list, its related states, country and zip code bind in respective dropdownlist.and textboxes.

Default.aspx code

<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>

</div>
<asp:Label ID="Label1" runat="server" Text="Choose city" Width="100px"></asp:Label>
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True">
<asp:ListItem>select</asp:ListItem>
</asp:DropDownList>
<br />
<asp:Label ID="Label2" runat="server" Text="State" Width="100px"></asp:Label>
<asp:DropDownList ID="DropDownList2" runat="server">
</asp:DropDownList>

</form>
</body>
</html>

Default.aspx.vb code

Imports System.Data
Imports System.Data.SqlClient
Partial Class _Default
Inherits System.Web.UI.Page
Dim strConnString As String = System.Configuration.ConfigurationManager.ConnectionStrings.Item("ConnectionString").ToString()
Dim con As New SqlConnection(strConnString)
Dim str As String
Dim com As SqlCommand
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not IsPostBack Then
con.Open()
str = "select * from area"
com = New SqlCommand(str, con)
Dim reader As SqlDataReader = com.ExecuteReader
While reader.Read
DropDownList1.Items.Add(reader("city"))
End While
reader.Close()
con.Close()
End If
DropDownList2.Items.Clear()
DropDownList3.Items.Clear()
TextBox1.Text = ""
End Sub

Protected Sub DropDownList1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged
con.Open()
str = "select * from area where city='" & DropDownList1.SelectedValue & "'"
com = New SqlCommand(str, con)
Dim reader As SqlDataReader = com.ExecuteReader
Do While reader.Read
DropDownList2.Items.Add(reader("state"))
DropDownList3.Items.Add(reader("country"))
TextBox1.Text = reader("stdcode")
Loop
reader.Close()
con.Close()
End Sub
End Class

Output