error>> network on main thread exception

package com.example.bms;

import java.io.InputStream;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.List;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class Add_Customer extends Activity{
	EditText customerName,addr1,addr2,person1,mob,pos,note;
	Button Save,cancel;
	List<namevaluepair> nameValuePairs;
	InputStream is;
	DateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");
	Calendar cal = Calendar.getInstance();
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		// TODO Auto-generated method stub
		super.onCreate(savedInstanceState);
		setContentView(R.layout.add_customer);
		customerName=(EditText)findViewById(R.id.CustomerName);
        addr1=(EditText)findViewById(R.id.addr1);
        Save=(Button)findViewById(R.id.save);
        cancel=(Button)findViewById(R.id.cancel);
        addr2=(EditText)findViewById(R.id.addr2);
        person1=(EditText)findViewById(R.id.Person);
        mob=(EditText)findViewById(R.id.Mobile);
        pos=(EditText)findViewById(R.id.Position);
        note=(EditText)findViewById(R.id.tNote);
        		
		  Save.setOnClickListener(new OnClickListener() {
			
			@Override
			public void onClick(View v) {
				try{
			        HttpClient httpclient = new DefaultHttpClient();
			        HttpPost httppost = new
			        HttpPost("http://123.12.11.1/insertCustomer.php");
			        nameValuePairs = new ArrayList<namevaluepair>(3);
			        nameValuePairs.add(new BasicNameValuePair("cName", customerName.getText().toString()));
			        nameValuePairs.add(new BasicNameValuePair("addr1", addr1.getText().toString()));
			        nameValuePairs.add(new BasicNameValuePair("addr2", addr2.getText().toString()));
			        nameValuePairs.add(new BasicNameValuePair("person1", person1.getText().toString()));
			        nameValuePairs.add(new BasicNameValuePair("mob", mob.getText().toString()));
			        nameValuePairs.add(new BasicNameValuePair("pos", pos.getText().toString()));
			        nameValuePairs.add(new BasicNameValuePair("note", note.getText().toString()));
			        nameValuePairs.add(new BasicNameValuePair("date", dateFormat.format(cal.getTime())));
			        UrlEncodedFormEntity URLEntity = new UrlEncodedFormEntity(nameValuePairs);
			        httppost.setEntity(URLEntity );

			        HttpResponse response = httpclient.execute(httppost);
			        HttpEntity entity = response.getEntity();
			        is = entity.getContent();
			        int status = response.getStatusLine().getStatusCode();
			        if(status==200)
			        	 Toast.makeText(Add_Customer.this,"New Customer Information added,Successfully !", Toast.LENGTH_LONG).show();
			        else Toast.makeText(Add_Customer.this,"Customer Information Not Saved, Error in http Connection: ", Toast.LENGTH_LONG).show();
					
			        }catch(Exception e){
			        
			        Toast.makeText(Add_Customer.this,"Error in http Connection: "+e.toString(), Toast.LENGTH_LONG).show();
			        }				
			}
		});		
	}
}