Multithread telnet not working Parallel

[anna]

#!/usr/bin/python
import threading
import sys
import os
import unidecode
import telnetlib
import time
import re
from time import sleep
outfile = open('zyxelmac.txt','w')
def open_telnet(ip):
        user = 'admin'
        password = '1234'
        sr_no = 0
        try:
            telnet = telnetlib.Telnet(ip, 23, 2)
            telnet.read_until('User name:', 3)
            telnet.write(user.encode('ascii') + '\r')
            telnet.read_until('Password:', 3)
            telnet.write(password.encode('ascii') + '\r')
            telnet.write('statistics mac 1~48' + '\r\r\r\r')
            telnet.read_until('>')
            telnet.write(' exit''\r')
            output = telnet.read_all()
            sleep(2)
            data = iter(output.split('\n'))

            try:
                        for line in data:
                                if "Port:" in line:
                                        port1 = line.split(':')
                                        port = str(port1[:5])
                                        port2 = port[10:13].strip()
                                        port3 = port2.replace('\\','')
                                        next(data)
                                        next(data)
                                        port_mac = next(data)[1:]
                                        outfile.write(ip+' '+port3+' '+port_mac)
                                        outfile.write('\n')
            except StopIteration:
                   pass
        except Exception as excp:
            print(excp)
def create_threads():
    threads = []
    with open('zyxel.txt','r') as ipfile:
        for sr_no, line in enumerate(ipfile, start=1):
            ip = line.strip()
            th = threading.Thread(target = open_telnet ,args = (ip,))
            th.start()
            threads.append(th)
            for thr in threads:
                thr.join()

if __name__ == "__main__":
        create_threads()
        print "Exiting the program"
        outfile.close()

above script is working sequentially and not Parallel. please suggest.